The United States sank or crippled six Iranian ships and fired at Iranian warplanes yesterday during a daylong series of fierce sea and air battles that erupted across the Persian Gulf after the U.S. Navy destroyed two oil platforms in a retaliatory strike ordered by President Reagan.

Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, told a Pentagon news conference while the battles were tapering off that the United States had suffered no losses of men or ships and that Iranian casualties were heavy. Last night, a Marine Cobra helicopter gunship with a two-man crew was missing from the cruiser USS Wainwright and a search was under way. Iran claimed it had shot the chopper down, but Pentagon sources had no information on the cause of its disappearance.

Crowe said he did not know how many sailors Iran had lost. U.S. estimates of Iranian ship losses as of last night were one Combattante II high-speed missile boat sunk; one Boghammar patrol boat sunk and two others believed crippled, and two Vosper Mark 5 frigates severely damaged, if not sunk.

President Reagan said, "We aim to deter further Iranian aggression, not provoke it. They must know that we will protect our ships, and if they threaten us, they'll pay a price." There was wide bipartisan approval in Congress of the president's action. {Details on Page A23.}

The clashes added up to the biggest gulf naval engagement since the United States deployed ships there last year to escort Kuwaiti oil tankers in the vital sea lanes. It also marked the sharpest hostilities between the United States and Iran since the fall of the shah in 1979.

Destruction of the oil platforms was a modest economic blow to a major petroleum power, and Iran reacted with unexpected violence, not only attacking U.S. Navy units but also launching attacks against three civilian tankers and a commercial oil platform with many American workers on board.

{In response to the hostilities, oil prices on world markets jumped yesterday to their highest levels of the year. Details on Page C1.}

The losses inflicted by the United States came the same day that Iran also was reportedly dislodged from Iraq's strategic Faw Peninsula, which Tehran's ground forces had held for two years.

At one point during the sea battles on the gulf, the United States and Iran were firing U.S.-made Harpoon antiship missiles at each other's vessels. At another, two F4 fighter-bombers, which the United States had sold the shah when Iran was an ally, dove from the clear sky and fired missiles at the Wainwright, a guided missile cruiser.

Despite all the gunfire on the gulf, Reagan and Defense Secretary Frank C. Carlucci said the United States was not trying to provoke Iran, but was carrying out limited military retaliation for the Iranian mine that almost sank the USS Samuel B. Roberts, a guided missile frigate, last Thursday. The Roberts struck the mine after escorting a Kuwaiti tanker flying the U.S. flag through the gulf.

"Our Navy made a measured response to Iran's latest use of military force against U.S. ships in international waters as well as its continued military terrorist attacks against a number of nonbelligerents," Reagan said. "We've taken this action to make sure the Iranians have no illusions about the cost of irresponsible behavior. We aim to deter further Iranian aggression, not provoke it. They must know that we will protect our ships. And if they threaten us, they'll pay a price."

"These strikes have been launched in retaliation for the damage done by an Iranian mine to the USS Samuel B. Roberts," Carlucci said at a morning Pentagon news conference. "The government of Iran fully knew and understood the potential consequences of its actions . . . . We regard the mission as having been accomplished . . . . "

Behind those statements, administration and military leaders said, was a reaffirmation by Reagan of U.S. tit-for-tat policy in the gulf, which calls for the United States to respond militarily in a proportional, limited way to hostile acts by Iran.

Backers of this policy contend that it has brought results, pointing to the long suspension of Iranian mining after the United States captured the Iran Ajr in the act of laying mines last September. A subsequent U.S. attack on an Iranian oil platform used as a military communications center was retaliation for Iran's Silkworm missile attack on a Kuwaiti tanker flying the U.S. flag.

Opponents of tit-for-tat argue that it failed during the Vietnam war as North Vietnam adjusted to the pain. They favor a much tougher retaliatory policy, such as strikes to wipe out the Iranian navy or destroy its big naval base at Bandar Abbas. Strikes against bigger targets have been mapped as part of the U.S. military's contingency planning in the gulf, officials said.

The wide-ranging naval and air action yesterday began just after 9 a.m. (1 a.m. EDT) when U.S. Navy forces launched virtually simultaneous attacks on two oil drilling platforms in the southern gulf used by the Iranians to monitor ships and provide military communications.

The attacks set off a chain of confrontations between U.S. and Iranian ships and planes for nine hours across a 400-mile stretch of the gulf. Navy warships and fighter planes from the carrier USS Enterprise used some of their most sophisticated weapons to cripple aging Iranian frigates and small speedboats.

By twilight in the gulf, U.S. and Iranian forces had clashed in five separate incidents that sank at least two Iranian patrol boats, severely damaged two others, crippled two Iranian frigates and may have damaged one Iranian fighter plane, Pentagon officials said. They also reported scattered firings on several U.S. helicopters throughout the central and southern gulf area.

"Our initial mission has been accomplished," Carlucci declared after darkness fell over the Persian Gulf and the fighting subsided.

The initial mission involved the destruction of the two oil platforms -- both larger than the Rostam oil platform destroyed and left burning last fall in retaliation for the Iranian Silkworm missile attack on a U.S.-flagged Kuwaiti tanker. This mission began as U.S. warships closed in on the Sassan platform, a steel-girdered base for military communications and surveillance.

"We had three ships {destroyers USS Merrill and Lynde McCormick and landing ship USS Trenton} at the Sassan platform," Crowe said yesterday. "About 1 a.m. {EDT}, 9 a.m. their time, we gave a warning to the platform that we would be destroying it and asked them to abandon. They fired at us."

The Iranian attempt to defend was a sharp contrast to last year's attack, when the crews aboard the Rostam platform quickly evacuated.

Crowe said the Iranians fired Soviet-made ZSU23 automatic weapons at the ships but apparently caused no damage. They ceased firing and the U.S. ships "fired a few shots" with their five-inch guns, prompting some of the Iranians to clamber into a boat to escape.

"When the boat pulled away it looked like nobody else was abandoning," Crowe said. "We fired some more shots. They returned fire, but they again made moves to abandon."

The U.S. warships stopped firing until they believed everyone had left the platform, fired a few more rounds "to make sure it was safe," then sent a contingent of Marines onto the platform to blow it up, Crowe said.

The Marines found four of the automatic weapons and several SA7 antiaircraft missiles before they destroyed the platform, leaving a smoking mound of twisted metal.

About 100 miles away, three more warships -- frigates USS Simpson and Bagley and the Wainwright -- were issuing the same warnings to Iranians on the Nasr platform off Sirri Island. Although Navy personnel saw a gun manned on the platform, it was not fired at U.S. vessels, officials said.

Most of the Iranians on the second platform fled after initial warnings, but a few remained.

"We then took the platform under fire," said Crowe. "They then went into the water. We do not know if any of those people were killed or not." When an Iranian tug appeared, the Wainwright ceased fire for a few minutes and allowed it to rescue Iranians flailing in the water.

Although the Navy intended to have a Marine demolition team blow up the platform, the working oil rig portion of the platform was blazing so fiercely that they could not approach it. The Wainwright destroyed the platform with 1,000 rounds of five-inch gunfire.

About 1 1/2 hours later, Iranian ships, small craft and fighter planes stepped up attacks throughout the gulf. At 12:15 p.m. (4:15 a.m. EDT), three Iranian speedboats fired on a helicopter from the Simpson near the Nasr platform, officials said. The helicopter, which did not have the weapons to return fire, reported back to its mother ship with no damage, officials said.

Pentagon officials said there were several other reports of American helicopters being fired on throughout the day, but could not confirm that any had been hit.

Shortly after the attack on the Simpson's chopper, the U.S. commercial supply ship Willi Tide was reported under fire in the Mubarak oil field to the southeast. Officials reported no one killed in the attack, and the ship apparently suffered no severe damage.

Minutes later, the Iranian missile patrol boat Joshan began closing in on the Wainwright. U.S. officials said the American ship radioed three warnings, but the Joshan, a French-made Combattante II missile fast attack craft, ignored the warnings and moved within 10 miles of the U.S. ship.

"We immediately informed it on the air that it was to stay clear, that it was to reverse course and not interfere," Crowe said. "It acknowledged the message, but continued to close."

The two vessels fired missiles at each other almost simultaneously, officials said.

The Wainwright's crew foiled the Iranian missile by firing off clouds of metallic chaff, which confuses the missile's guidance system. The Wainwright's missile smashed into the Iranian craft, along with two of three missiles fired by the Simpson. A fourth missile hit the vessel and sank it in a blaze of fire and smoke. Officials said they do not know if the vessel's crew -- estimated at 31 -- was killed or rescued by other small boats in the area.

Within the hour, two Iranian F4 fighter planes streaked toward the Wainwright. The planes dove and reportedly fired a missile that missed its target, officials said. The Wainwright fired back with antiaircraft missiles. Pentagon officials said both planes immediately turned and returned to their home bases. Officials said one plane appeared to have been hit, but did not crash.

The next report of an attack came about 30 minutes later, when a Panamanian oil rig with 16 Americans aboard was reported under attack by an Iranian helicopter.

Meanwhile, the U.S. military intercepted communications in which Tehran ordered two of its frigates, the Sahand and the Sabalan, to retaliate against U.S. naval forces. The 310-foot British-made vessels and two sister ships are among Iran's largest surface warships.

With the approach of the Iranian vessels, U.S. commanders dropped plans to attack a small third platform to the north at Rakash, and moved into position to blockade the Iranian warships, which have been used frequently by Iran to harass and fire upon commercial shipping in the gulf, according to Pentagon officials.

In the same area, pilots in Navy A6 Intruder attack planes flying combat patrols spotted three Iranian Boghammar speed boats firing on other small boats in the oil field area, officials said.

The U.S. pilots fired on the Boghammars, sinking one and disabling another, officials said. The fate of the third boat was unknown.

Navy ships and planes then began battling the two Iranian warships.

The Sahand moved toward the U.S. ships and was warned away at 3:59 p.m. (7:59 a.m. EDT). It responded 22 minutes later by opening fire on the U.S. ships. American A6 planes hit the Iranian ship with a Harpoon missile, then blasted it seven minutes later with laser-guided bombs. The frigate USS Strauss then scored a direct hit with another Harpoon.

The Sahand was left with its superstructure in flames and a large hole in its starboard side. U.S. officials said they do not know how many crewmen were killed but reported several life boats in the water near the burning vessel.

Two hours later, the Iranian frigate Sabalan fired a surface-to-surface missile at the frigate USS Jack Williams and a second missile at the Navy attack planes flying overhead. Both missed their targets.

"We hit it with a laser-guided bomb," said Carlucci. "We did not follow up because the ship looked to be crippled and it had two tugs alongside and was clearly taking water."

The bombs from the A6 Intruders left the Sabalan taking on water with the stern partially submerged, officials said.

Carlucci described the Iranian attacks on the U.S. ships and aircraft as "foolhardy," and called them uncharacteristic of the Iranians, whose ships are smaller and more lightly armed than the U.S. warships. "It certainly makes no sense whatsoever in military terms," he said.

Last night, Carlucci said, "Hostilities have ended, unless the Iranians once again demonstrate hostile intent."