KUWAIT (AP) _ Three American-flagged Kuwaiti tankers docked in their home port to begin loading oil today after successfully navigating the Persian Gulf with their U.S. Navy escorts.

A new mining threat outside the waterway prompted France and Britain to dispatch minesweepers.

A fifth mine was reported found today south of the Strait of Hormuz in the Gulf of Oman, previously considered mine-free and safe.

United Arab Emirate officials told The Associated Press two of the five mines were defused by United Arab Emirate coast guardsmen. The officials in Abu Dhabi, speaking on condition of anonymity, said the mines were picked up by patrol boats in United Arab Emirate waters about nine miles off the port of Fujairah. The area is not far from where a U.S.-operated supertanker, the 274,347-ton Texaco Caribbean, was damaged by a mine on Monday.

Earlier reports said four mines had been spotted in the busy coastal anchorage, some 30 miles south of the Strait of Hormuz, gateway to the Persian Gulf. U.S. Navy and Omani helicopters and Omani coast guard vessels carried out the search for the floating explosives.

In Washington, Defense Secretary Caspar Weinberger said the United States was seeking support for an international minesweeping operation in the gulf ''in the interest of all maritime nations.'' But France and Britain said their sweepers were sent to protect their own vessels.

Iran's ambassador to the United Nations, Said Rajaie-Khorassani, said his country would cooperate with U.N. peace efforts but added Tehran was not ''accepted or rejecting'' a Security Council resolution calling for an end to the 7-year-old Iran-Iraq war.

U.N. Secretary-General Javier Perez de Cuellar said Iraq had accepted the resolution.

The Gas King, Sea Isle City and Ocean City left their U.S. escorts and arrived in port Tuesday, ending the second Navy-escorted trip through the gulf.

This morning, reporters saw the 46,723-ton Gas King and the 79,999-ton Ocean City docked at Al-Ahmadi, Kuwait's main oil terminal, to begin the roughly five-day loading process.

The 81,283-ton Sea Isle City rested at anchor about a mile offshore, and a shipping source, speaking on condition of anonymity, said it would dock later today. The Gas King is to take on liquefied petroleum gas, while the other two will be loaded with naphtha and refined oil.

Also at anchor offshore this morning was the supertanker Bridgeton, which hit a mine on the initial U.S.-escorted convoy.

Shipping sources said the tanker, carrying a partial load of crude oil despite a gaping hole in its hull, was likely to accompany the three other tankers on their return convoy. No date has been announced for that trip.

Eleven of Kuwait's 21 tankers are being registered as American ships so the U.S. Navy can protect them from attacks by Iran, which accuses Kuwait of supporting Iraq in the gulf war.

On Saturday, an American fighter fired two missiles at an Iranian jet after the jet ignored radio messages to steer away from a U.S. surveillance plane, Pentagon sources said. The missiles missed.

A U.S. Navy Aegis-class cruiser, the Valley Forge, and a U.S. Navy P-3 Orion surveillance plane tracked the Iranian F-4 Phantom starting from its takeoff at Iran's Bandar Abbas naval base, the sources said, speaking on condition of anonymity.

The confrontation occurred over the Strait of Hormuz when the Iranian jet began flying directly toward the unarmed P-3 aircraft, they said.

Asked Tuesday about Tehran's reaction to the incident, Rajaie-Khorassani told ABC News: ''I think it has been something very casual. My personal reaction is that if we fire we're not going to miss, so I advise the Americans not to do it again.''

Pentagon sources in Washington said the U.S. helicopter carrier Guadalcanal and its eight Navy mine-sweeping helicopters will not arrive in the Persian Gulf until next week because of problems with the Guadalcanal's electrical system.

The Soviet news agency Tass said the U.S. request for an international mine-detecting force was designed to convince Americans that Washington was not politically isolated in its reflagging operation.

Tehran radio quoted Hashemi Rafsanjani, speaker of Iran's Parliament, as saying the Iranian navy would be sent to the Gulf of Oman to clear mines, but there was no elaboration.

The anchorage, 30 miles from the Strait of Hormuz, is being used as a staging point for the U.S.-Kuwaiti convoys.

The Reagan administration said it agreed to the reflagging to protect the flow of oil in the Persian Gulf. It acted after the Soviets leased three tankers to Kuwait.