KUWAIT -- After a shaky beginning, the Reagan administration's six-month-old naval deployment has lifted American credibility in the Persian Gulf, in the view of many officials in the region, securing safe passage for a portion of Kuwait's petroleum exports under the U.S. flag.

But Arab leaders and western officials interviewed in gulf capitals warn that U.S. policy in coming months faces a critical test of whether it can contain the still rising level of violence in the "tanker war" between Iran and Iraq.

As Defense Secretary Frank Carlucci returns to Washington to report to President Reagan on a 10-day inspection tour of the U.S. fleet and a round of consultations with gulf and European leaders, a number of Arab officials are pressing the United States to expand its mission to protect all neutral shipping that transits the vital Strait of Hormuz from Iranian gunboat attacks.

Carlucci, in public statements during his tour, appeared to discount an expanded U.S. role and rejected any suggestion that the United States should become the "policeman of the high seas."

Yet some U.S. fleet commanders have told western officials that they want to be "unleashed." They say they have the forces and capability to intervene and stop Iranian gunboat aggression along the 550-mile waterway, steps that could put the United States in direct confrontation with Iran and probably prompt another war powers debate in Congress during this presidential election year.

For the moment, Washington appears intent on limiting the U.S. commitment to escorting 11 reflagged Kuwaiti tankers while separately pursuing a U.N. Security Council consensus to enforce its July 20 cease-fire resolution. Debate on that resolution has entered its most difficult phase at U.N. headquarters in New York, where council members must win votes from the Soviet Union and China to impose an arms embargo against Iran for failing to abide by the resolution's cease-fire call.

Thus, U.S. policy remains heavily affected -- or "trapped" some critics charge -- by parallel requirements to find a diplomatic solution to the land war while the U.S. Navy perseveres in its policeman's role in the gulf. Meanwhile, the seaborne guerrilla war appears certain to flare up periodically, threatening to render the American success irrelevant.

The year that marked America's involvement on the margins of the bloody Iran-Iraq conflict has been full of reversals and surprises for U.S. policymakers, so much so that one western diplomat said the American initiative gave new meaning to the old phrase to have "a life of its own."

A flexible and clever mix of Iranian tactics, with antiquated or makeshift weapons systems, quickly stripped away American confidence that "showing the flag" in the gulf would quell the violence and "contain the war," as the gulf U.S. naval commander, Rear Adm. Harold J. Bernsen, characterized the U.S. objective last summer.

But after the embarrassment of the Bridgeton, the reflagged supertanker that struck a hidden Iranian mine on July 24 during the U.S. Navy's maiden convoy operation, U.S. commanders quickly adapted to belatedly recognized threats. Today they are operating an innovative array of sea defenses, surveillance patrols and intelligence capabilities that have forced the Iranians to change tactics and abandon attacks in many areas of the gulf.

Nonetheless, the attacks on both sides in the seven-year-old war continue.

With weapons as sophisticated as French-made Exocet missiles and Chinese-made Silkworms, or as simple as machine guns and shoulder-fired rocket grenades, Iraq and Iran together last year tallied 178 strikes against commercial shipping, the largest single-year total. The death toll reached 108 merchant seamen. There were 80 ship attacks in 1986, killing 52 persons.

Although a lull has settled over the gulf combat zone in the opening days of the new year, a number of officials say Iran has charted an unrelenting challenge to the presence of American and western naval forces by concentrating its efforts on blasting holes in neutral-flag vessels -- often within sight of warship crews powerless to exceed standing orders to protect only ships of their national flag. Iranian attacks usually inflict only minor damage on mammoth crude carriers, but December's attacks sent the Singapore-registered Norman Atlantic, to the bottom and so heavily damaged two others, the Ariadne of Sweden and the Hyundai No. 7 of South Korea, that salvage executives considered them total losses.

At year's end, writeoff losses in the tanker war stood at nearly half the tonnage sent to the bottom during World War II, according to the Oslo-based International Association of Independent Tanker Owners.

"I think the feeling here is that something is going to have to be done," said a longtime U.S. analyst who lives in the region and has recently conferred with both Arab leaders and U.S. Navy officials.

If the president and his national security advisers continue to limit the U.S. role to its current U.S. flag escort mission, "then Iran has won because it has countered the convoys and the United States has failed to respond." In that event, he added, "the view is here that it would have been better not to have the convoys."

Yet other Arab and western officials credit the U.S. and western presence with preventing Iran's total domination of the sea lanes.

"Without the reflagging operation, I'll bet you the gulf would be an Iranian lake today," said a prominent Kuwaiti with close ties to the government. Another longtime western diplomat in the southern gulf credited the U.S. and western navies with preventing the strangulation of the waterway by extensive Iranian mine warfare last summer. Confronted by the mine attack on the Bridgeton, "the United States made it clear that if the Iranians put mines in the water, they were going to get hit," the diplomat said.

In the northern gulf, Kuwaiti officials have expressed strong satisfaction that U.S. warships and mine-sweepers have cleared the approaches to Kuwait's vital oil-loading port and virtually neutralized the speedboat attacks that were emanating from Iran's Farsi Island base. Western officials say Farsi's speedboat forces are all but "neutralized" after U.S. helicopter gunships responded to shots fired in their direction by opening up with Gatling guns on one speedboat flotilla the night of Oct. 8.

Ever since, "American helicopters have been sitting on those speedboats" from Farsi, said a European ambassador. "They are never alone."

Yet the large expanses of the southern gulf remain an Iranian shooting gallery for their retaliatory raids against neutral shipping.

Arab leaders in Saudi Arabia and the United Arab Emirates, whose shipping approaches are now most frequently under attack, expect the West to do more.

"If you say that the only role for a major power like the United States is to protect 11 commercial ships belonging to one country {Kuwait}, then in the long term that is bad for the credibility of the United States," an Arab government official said. "But if the American presence here is part of a policy to protect the vital interests of the West, to maintain the flow of oil, help contain the violence and bring an end to the war, this is what people are looking for."

The U.S. deployment to the gulf and adjacent waters outside the Strait of Hormuz, the largest massing of U.S. naval forces since the Vietnam War, has cost an estimated $120 million in the first six months and has resulted in 23 successful convoys. Each successful escort passage of U.S. warships and reflagged Kuwaiti tankers, plus occasional U.S. supply ships, costs more than $5 million.

The U.S. naval buildup, which now seems certain to be pared down for cost and efficiency reasons, reached a peak of 48 ships this winter, manned and supported by an estimated 25,000 U.S. servicemen in the Navy, Army and Marines. Allied naval forces from Britain, France, Italy, Belgium and the Netherlands have contributed 34 combat ships, mine-sweepers and support vessels for a total western force of 82 vessels deployed in the Persian Gulf or the adjacent Gulf of Oman, Arabian Sea and Indian Ocean.

The Soviet Union, which escorts not only cargo ships carrying war materiel to Iraq but also three oil tankers leased to Kuwait, has built up its naval presence in the region to an estimated 23 combat ships, mine-sweepers and support ships.

The management of U.S. gulf policy is under growing pressure from domestic forces as varied as cost-cutters in the Defense Department and congressional critics of Reagan's use of his war powers authority and because of the volatility of presidential election politics.

Nevertheless, western ambassadors, Arab officials and longtime analysts in the gulf say the United States has achieved significant strategic and diplomatic objectives by carrying out the convoy operation in a sustained and carefully calibrated manner.

They say the convoys continue to demonstrate U.S. commitment to moderate Arab states, have won support and naval cooperation from European allies who were not consulted at the outset and have denied the Soviet Union a significant role in the strategic waterway.

"If the Americans left and the Kuwaitis felt threatened, I'm convinced they'd bring in the Russians, or try to," said a senior shipping official here.

In addition, U.S. staying power and convoy management have engendered close working relations with Arab governments previously unwilling to cooperate with the United States after a legacy of disappointments over U.S. attitudes in the Arab-Israeli dispute and planned weapons sales to friendly Arab regimes that were scuttled by Israel's supporters in Congress.

And importantly, the U.S. initiative has done much to mollify the deep distrust and anti-American sentiment resulting from the revelations of secret U.S. arms sales to Iran.

Even with these results, the American deployment here remains untested, in the view of many officials, who say they fear that the loss of American lives could quickly wither U.S. resolve.

"Sooner or later, American sailors are going to die again in the gulf, and that will be a very critical point for the American commitment," said a European ambassador.

The deaths last May 17 of 37 sailors aboard the frigate USS Stark have haunted U.S. military commanders who have found their forces threatened by both accidental and intentional attacks from Iranian and Iraqi forces.

Some American naval commanders have expressed frustration over their limited role in a war zone that they strongly insist they can police with no additional resources.

"The instinct of the U.S. Navy is not to sit there and be a Quaker witness to these attacks -- it is to intervene," said a longtime American analyst in the region.

As U.S. sailors and their commanders came ashore for Christmas leave, frustration was reported running high in the fleet.

Crew members from the guided missile frigate USS Gallery recounted to western sources their Thanksgiving Day disappointment when they were prevented from stopping a gunboat attack on the Kuwait-registered tanker Umm Jathathel, which was trying to slip into the gulf disguised as the Romanian tanker Dacia.

The Gallery had been on escort duty with three other American warships passing through the Strait of Hormuz when it spotted the Kuwaiti ship with an Iranian frigate in hot pursuit. The Kuwaitis had failed to fully conceal the ship's original name by painting over it the name "Dacia," and the Iranian gunboat seemed bent on attack.

The Gallery's commander brought his warship alongside the Kuwaiti vessel just as the Iranian frigate pulled up on the opposite side and trained its four-inch gun on the defenseless tanker.

"The Iranian commander knew that his guns were not only trained on the tanker, but on the Gallery, and if he opened fire, under the rules of engagement the Gallery could return fire," a western source said. "The Gallery stayed there for a long time," the source said, preventing the Iranian attack.

But eventually, the U.S. warship was ordered by the U.S. Middle East Force to disengage. When the Kuwaiti tanker captain realized that the Gallery was pulling away, he called plaintively on the radio: "Gallery, Gallery, where are you going?"

"Sure enough, as soon as the Gallery left -- boom," said a western source to whom crew members complained about the incident. "As you can imagine, it was quite a morale-downer for the Gallery's crew," the western source added.

"And if you talk to the ship commanders, you'll find out how much they want to be unleashed," he said.

Western sources said British and American warships during the past two months have become more active in unofficially trying to extend the umbrella of warship protection to neutral ships that come under Iranian attack, but the American attempts have been largely limited to providing helicopter rescue and other humanitarian assistance to tanker crews after Iranian gunboats have peppered them with artillery shells and machine-gun fire.

"The British are doing more," said a Kuwaiti shipping official. "They have actually confronted the Iranians and told them over the radio, 'Leave that ship alone,' " when nearby a neutral-flag vessel was under threat of attack.

"But we haven't seen any indication that the Americans have done anything to stop an attack," the official said. "On the contrary, we have seen them stand by and let the Iranians have a free hit."

Senior Navy officials in Washington have argued that the United States would have to deploy even more naval resources to the gulf to extend the umbrella of protection to neutral shipping, but warship commanders have asserted to western officials in the region that they could broaden their mission using existing resources by simply putting Iran on notice that U.S. warships will intervene in any attack on neutral shipping.

"These guys are very confident that they can protect the convoys -- and neutral shipping," an American resident of the region said. "The Iranians are out there rubbing their noses in it, and they don't like it -- it's a very visceral reaction. As one of them put it, 'With the firepower we have here, this could all be over in 24 hours.' "

But Washington's interest in a measured response to Iran, meant to avoid further provocation and preserve the prospect for U.S.-Iranian relations after Khomeini's death, applies equal pressure to limit the American naval role in the gulf, according to western officials.

U.S. officials made a point after the Navy's Oct. 19 attack on Iran's speedboat base at Rostam platform that they had chosen a "measured response" to Iranian threats against U.S. interests in the waterway.

After Iran's Silkworm missile attack on the U.S.-flag tanker Sea Isle City, then-Defense Secretary Caspar W. Weinberger advocated destroying all Iranian island bases and platforms in the gulf used for attacks on shipping, according to a knowledgeable U.S. official.

Adm. William Crowe, chairman of the Joint Chiefs of Staff, advocated destruction of the entire Iranian fleet used to attack neutral shipping, the source added.

However, Carlucci, who was then serving as the president's national security adviser, supported a "measured" option that won presidential approval: the destruction of a single Iranian speedboat base at the Rostam platform.

"The United States is not going to do anything that puts it in a directly confrontational situation with Iran," said an official in Kuwait.

With its 1,600-mile border with the Soviet Union and population of 48 million people, Iran remains the constant fixation of both regional powers and the superpowers jockeying for position in the gulf.

"I think Washington is waiting to give the U.N. a chance," said a long-experienced diplomat in the region. "But by spring, if something hasn't happened" to secure the heavily traveled sea lanes through the Strait of Hormuz, "the people will again look to the United States to act unilaterally."

In the meantime, few officials in the region see any likelihood that Iran will respond to any of the overtures from the United Nations or the six-nation Gulf Cooperation Council to delay a major offensive against Iraq.

Western officials with access to current intelligence from the 700-mile war front say Iran's logistical buildup is still under way. Some predict that the offensive has been delayed by effective Iraqi air attacks that have disrupted rear area supply lines and by the general deterioration of Iran's transportation system.

"Everybody hopes and hangs on the idea that Iran is going to negotiate. I don't believe it," said the diplomat. "In Tehran, spirits are high. They feel they have countered the convoys and found out that the giant {the United States} has feet of clay. They are in no worse shape than when the convoys began, and as long as {Ayatollah Ruhollah} Khomeini is alive, they are not going to negotiate."