MANAMA, BAHRAIN, SEPT. 24 -- The U.S. Navy concluded an agreement today to return to Iran -- through Oman -- 26 seamen who survived Monday's helicopter assault on their mine-laying vessel.

Meanwhile, shipping sources in the Persian Gulf reported that in a separate incident early Tuesday a small survey ship struck a mine north of here and sank, apparently killing four crewmen, before it could broadcast a Mayday signal.

The 180-ton Marissa I, used to survey shipping channels in the gulf, was registered to a Panamanian company and by late today no government or commercial concern along the waterway had announced who was operating the ship. Lloyd's of London, which first reported the sinking, said it had determined only that the ship was steaming from Kuwait to Dubai when it struck the mine.

At the United Nations in New York, the U.S. secretary of state and Soviet foreign minister agreed to defer temporarily a U.S.-backed drive for an arms embargo against Iran. {Details on Page A32.}

Oman's Foreign Ministry issued a statement today saying it welcomed the decision by the United States and Iran to repatriate the seamen of the Iran Ajr. It was attacked by U.S. Army helicopters specially equipped for night operations whose crews said they spotted the vessel laying mines in waters 50 miles northeast of here.

Western sources in the Persian Gulf said the Iranians would be flown Saturday by U.S. Marine helicopters from U.S. warships off Bahrain to Seeb airport in Oman, where they will board an Iranian Air Force plane for the trip to Tehran.

The bodies of three Iranians killed by rocket and machine-gun fire during the attack are expected to be transferred in the same operation. Two other members of the Iran Ajr crew are missing and presumed dead.

The Marissa I struck the mine at about 1 a.m. local time Tuesday 20 miles west of Farsi Island, in the same area where the reflagged Kuwaiti supertanker Bridgeton struck a mine during the first U.S. Navy escort mission on July 24.

Lloyd's said three of the seven-man crew survived the explosion and it listed four others as casualties. A Lloyd's spokesman said there was no other information about the fate or the nationality of the crewmen.

The sinking of the Marissa I followed a night of intense violence in the gulf, beginning with an Iranian gunboat attack on the British-flag tanker Gentle Breeze, also in the vicinity of Farsi Island, which is used by Iranian speedboat commandos for attacks on merchant shipping. One crewman was killed in that attack.

Little more than an hour later and farther south, U.S. Army helicopters fired upon and disabled the landing craft Iran Ajr, which Navy officials believe had dumped at least six contact mines in waters used by U.S. warships for anchorage.

U.S. mine-hunting forces believe that they have located all of the mines planted by the Iran Ajr before the U.S. attack, according to Pentagon sources. The sources said the crews of Navy helicopters and ships had located six or seven mines drifting in the area near the attack. The Pentagon officially has confirmed only that three of the mines have been found.

The mines found aboard the captured Iranian ship appeared to be produced in Iran, according to Pentagon sources. U.S. officials had believed that many of Iran's mines were purchased from North Korea. Iran said recently, however, that it has the capability to produce large numbers of mines in its own factories.

U.S. intelligence officials are sifting through documents and books found aboard the Iran Ajr, Pentagon sources said. Many of the documents aboard the ship apparently were destroyed before the crew abandoned it, according to reports filed by members of a media pool who toured the damaged vessel. They reported some documents had been shredded and others ripped into pieces.

Meanwhile, a British bomb disposal team arrived in Bahrain today to search the Gentle Breeze for unexploded grenades and rockets that were fired into its crew and machinery sections during the attack.

There were conflicting reports along the waterway today about the location of the 10th U.S.-escorted convoy of reflagged Kuwaiti tankers. While the Pentagon asserted that the natural gas carrier Gas Prince had not begun its voyage south to the Strait of Hormuz, shipping sources and news photographers aboard a helicopter reported seeing a U.S. naval convoy heading south at midday.

In the southern gulf, there were also reports that a convoy, either American or British, was heading into the Strait of Hormuz, but neither government confirmed the sighting, adhering to a policy of secrecy on convoy movements.

The tanker war has remained quiet since Monday's spate of attacks, and U.S. officials here prepared for Friday's arrival of Defense Secretary Caspar W. Weinberger, who will visit warship crews of the Middle East Force. A western official said Weinberger had landed in Saudi Arabia tonight but would not comment on his further travel plans.

Weinberger is scheduled to visit American ships in the gulf on Friday, then will meet with government leaders in Saudi Arabia and Bahrain over the weekend, according to Pentagon officials.

On the Iran-Iraq war front, Iraq's military command announced that its warplanes struck major electrical generating facilities at Dezful and Reza Shah, and Tehran acknowledged air strikes in the area.

Iran kept up a shelling barrage against Basra, where Iraq said 12 civilians had been killed and 49 wounded overnight.

In an interview with a Kuwaiti newspaper, Arab League Secretary General Chadli Klibi called on the Arab states to draw up a military strategy to end the war if a peaceful solution cannot be found.

"We prefer peaceful confrontation to stop the war, but if the war does not stop," he said, it will be up to the Arab states to reply jointly to the military threat to the region.Washington Post Pentagon correspondent Molly Moore contributed to this report in Washington.