MANAMA, BAHRAIN, NOV. 28 -- An unpublicized offer by Kuwait to allow the U.S. Navy, for the first time, to set up a big ocean-going barge as a floating naval base inside Kuwaiti territorial waters has been turned down in a surprise decision by the Reagan administration, according to a knowledgeable official in the region.

The U.S. military command in the Persian Gulf has made highly effective use of two barges elsewhere in the waterway as floating fortresses aimed at countering Iranian mine threats and speedboat attacks against U.S.-protected ships.

The command is known to have wanted such a facility inside Kuwaiti waters. As recently as last month, Rear Adm. Harold J. Bernsen, commander of the Middle East Force headquartered here, told western officials that a new naval base was planned for waters near Kuwait.

Early in the spring, after months of concentrated Iranian attacks against Kuwaiti shipping, the sheikdom's government approached the United States and the Soviet Union seeking protection against this sudden escalation of the seven-year-old Iran-Iraq war. The United States placed 11 Kuwaiti tankers under the U.S. flag and American naval protection, but Kuwait, asserting its sovereignty, has consistently refused to allow U.S. warships into its territorial waters.

In turning down Kuwait's offer of a naval facility, the administration apparently expressed new concerns about its ability to protect the barge and other targets inside Kuwaiti waters from Chinese-built, land-based Silkworm missiles that have been supplied to Iran and used successfully against Kuwaiti shore facilities and tankers. The missiles can hit targets up to 50 miles away.

More broadly, however, the change in Washington's position has caused some key officials in the Kuwaiti government to question whether the Americans, who in the past have expressed confidence about defending U.S. forces against Silkworms, are now trying to disengage themselves, citing security concerns, from any commitment to defend Kuwaiti territory from direct attack by Iran.

The importance of the Kuwaiti offer and the American response, in the view of a number of western and Arab officials in the region, is that it focuses attention on the distinction between the narrow U.S. military role here of escorting the reflagged Kuwaiti tankers and the much larger, implicit policy of containing the Persian Gulf war and protecting the moderate Arab states from Iranian aggression.

The Kuwaitis, the official said, offered to moor an ocean-going barge in territorial waters between Kuwait's main oil port and the entrance to the port channel. The facility, as big as a football field, would have enabled the U.S. Navy to position supplies, ammunition, spares, fuel and other stores critical to the growing logistical effort to service the U.S. warships involved in convoy operations.

"Earlier on, the Americans wanted to be in a better military position in the northern gulf," the official said. As U.S.-Kuwaiti talks on naval facilities continued this fall, Kuwait dropped its resistance to allowing U.S. warships and facilities in its territorial waters just as Iran escalated the gulf war with successful Silkworm strikes on targets inside Kuwait.

"The Americans pulled back," the official said, adding that Reagan administration officials "don't want to be put in a position where they've got to defend Kuwait should something happen," the official said.

"I don't believe anybody is going to defend Kuwait in a real crisis," he added.

The new concerns expressed by Pentagon officials about the vulnerability of U.S. sea bases raise questions about the overall vulnerability of U.S. forces potentially within range of mobile Silkworm batteries. The batteries could be deployed all along the 550-mile waterway in light of Iran's recently reported acquisition of a large quantity of Silkworms from China.

The official pointed out that during former defense secretary Caspar W. Weinberger's tour of the gulf in September, aides traveling with Weinberger instilled great confidence in Kuwaiti officials that Kuwait would not find itself alone if Iran staged a direct attack on Kuwaiti territory. One official in Weinberger's party told a reporter that if Kuwait is attacked, "We'll be there."

With Weinberger's resignation and replacement by Frank Carlucci, this official said, "I don't sense the same American commitment we had in the past due to the way they {U.S. officials} have been positioning themselves."

In successive weeks last month, Iran struck two oil tankers -- one flying the American flag and the other owned by U.S. interests -- and a key Kuwaiti oil loading terminal 10 miles off Kuwait's Ahmadi oil port.

The United States retaliated for the strike on the U.S.-flag vessel, but U.S. officials drew a clear distinction between that attack and the missile strike on Kuwait's Sea Island Terminal that Secretary of State George P. Shultz termed "an attack on Kuwait."

As of last month, the United States has had two operational sea bases in the Persian Gulf consisting of large ocean-going barges, also the size of football fields. A Senate staff report said both barges were "provided by Kuwait." The Navy has equipped the barges with attack helicopters, fast patrol boats, Army and Navy commando teams, long-range radar and intelligence gathering units that intercept Iranian communications, according to western sources familiar with the barge operations.

"It's all sandbagged," said one western military analyst. "It's a fort sitting in the middle of the gulf."

Western sources said the bases have extended American military power over vast portions of the strategic waterway, making up for the absence of U.S. land bases in nearby Arab states. The bases also have helped the Navy overcome operational limitations posed by shallow water and narrow channels that have made it impossible for U.S. aircraft carriers to maneuver inside the gulf.

One of the bases, code-named Hercules, is a converted North Sea drilling platform chartered from Brown & Root, the U.S. construction giant, shipping sources said. It is now stationed about 20 miles off Iran's Farsi Island, from which Revolutionary Guard commandos in speedboats and other craft had terrorized shipping in the northern gulf. Iranian forces on Farsi are believed responsible for sowing dozens of mines in shipping channels.

One such mine blew a hole in the Kuwaiti supertanker Bridgeton on July 24 on its maiden voyage under U.S. Navy escort.

"It's quite clear by now that Farsi Island has been completely neutralized," one western ambassador in the region said. "The Iranians are still there at Farsi, but they are never alone."

Shipping sources in the gulf and in Washington said three Iranian speedboats attacked by U.S. helicopters on Oct. 8 were "in the vicinity" of the American sea base. A knowledgeable gulf shipping source said the Iranian speedboats, aboard which six Iranians were fatally wounded after they reportedly fired in the direction of U.S. gunships, were "making a beeline" for the U.S. barge when they were intercepted.

Pentagon officials did not mention the Hercules base in their initial accounts of the incident.

Shipping officials in the region said the U.S. Navy is considering locating a new sea base in the southern gulf, near the Iranian speedboat enclave around the island of Abu Musa near the Strait of Hormuz, where Iran also has deployed Silkworm missile batteries.

Western sources said the Hercules base, whose helicopters and patrol boats have actively pursued Iranian speedboat sorties coming out of Farsi, is mobile -- by towing -- and is protected by the Phalanx antimissile system and portable Stinger antiaircraft missiles.

After a Senate Foreign Relations Committee staff group visited the gulf in September, it reported that with the barges in place, "Admiral Bernsen hopes, through vigorous patrolling, to prevent much of the mining" in the waterway.

Everything about the barge operations is being treated secretively to give the bases as little attention as possible amid the high-profile U.S. military presence in the gulf, sources said. Gulf-based supply companies that service the barges with generator fuel and stores are instructed to keep the barge locations secret.

To help protect the barges from the sea-going equivalent of the truck bomber that killed 242 Marines in Beirut in October 1983, the Navy is using trained "guard dolphins" to patrol the waters near the sea bases day and night, western sources said.

The Navy's deployment of six trained dolphins to the gulf was reported last month, but their role in detecting underwater saboteurs near the U.S. sea bases was not mentioned in a Pentagon statement.

"Personally, I'm a little worried about defending those things," said a senior western official. "They're a sitting duck." The official said the barges also highlight the ongoing arm's-length relationship between the U.S. military force in the gulf and the Arab gulf states that are the principal beneficiaries of U.S. policy in the region.

"What does it say about your relations with gulf states when you've got to operate from a barge out in the middle of the gulf?" the official asked.

Still, because of the size and compartmentalization of the barges, the official said, "you could hit them with a boatload of explosives and probably not do a hell of a lot of damage to them."