U.S. Navy commandos yesterday boarded and captured the Iranian navy ship that was attacked by American helicopters Monday in the Persian Gulf, and 26 surviving crew members who had abandoned the mine-carrying vessel were rescued by U.S. forces, Defense Department officials said.

A new incident occurred yesterday when the U.S. frigate involved in the attack fired warning shots at an Iranian hovercraft as it sped toward U.S. warships gathered near the disabled Iranian vessel, officials said.

Three Iranian sailors were killed in the Monday attack on the amphibious landing ship Iran Ajr about 50 miles northeast of Bahrain and two remained missing yesterday, according to Pentagon officials. Four of the Iranians rescued by U.S. naval forces were wounded, officials said.

President Reagan yesterday defended the U.S. attack on the ship as "authorized by law" because the vessel was laying mines in international waters.

A group of American journalists who were brought by the Navy to tour the captured ship reported seeing large contact mines sitting on the deck of the Iran Ajr. Military officials said the U.S. attack was ordered when the helicopter crews, using sophisticated night-vision equipment, spotted the ship's crew dropping mines overboard.

Some congressional leaders said the attack, the first U.S. military action in which Iranians are known to have died since U.S. Navy warships began escorting reflagged Kuwaiti tankers, have raised new concerns about the U.S. operation and the 1973 War Powers Resolution. {Details on Page A26.}

To avoid a confrontation over the resolution, which requires congressional approval of sustained U.S. combat operations, Senate leaders last night were studying a compromise that would require congressional approval to continue the escort operation after a specified period of time.

White House officials said administration lawyers have concluded that the gulf operations have not reached the point where congressional approval is required.

Pentagon spokesman Fred S. Hoffman said, "We don't consider this an escalation. This was an act of self-defense." He added, "This is the first time we caught them {Iranians} red-handed" laying mines.

Iranian officials in Tehran and at the United Nations in New York called the U.S. version of the incident "a pack of lies" and vowed the attack would not go unanswered.

Administration officials said the United States plans to turn the surviving sailors over to Iran as soon as possible, probably through an international Red Cross agency such as the Saudi Arabian-based Red Crescent. Three survivors are undergoing medical treatment aboard the amphibious assault ship USS Guadalcanal and the remainder are being held aboard the command ship USS LaSalle, officials said.

Defense Department officials said the crippled Iranian ship is being towed to another location in international waters by the guided missile frigate USS Jarrett, whose helicopters carried out the Monday attack. White House spokesman Marlin Fitzwater said the captured ship's "final disposition has not been determined."

Pentagon officials said the hovercraft incident occurred about 10:30 a.m. EDT yesterday when the Jarrett fired warning shots. Officials said the Iranian hovercraft, a high-speed vessel that skims the water on a cushion of air, had been cruising near the Jarrett and the LaSalle for about an hour, sometimes moving within less than a mile of the warships.

Ignoring radio warnings from the Jarrett to keep away, the hovercraft continued racing toward the ship, which fired two warning shots. The hovercraft turned away and stopped, officials said.

Pentagon sources said yesterday that the Iran Ajr and similar Iranian amphibious landing ships have been under surveillance by U.S. military and intelligence forces, who consider them the most probable source of Iranian mine-laying in the Persian Gulf.

Army special operations helicopters, equipped with night-vision equipment, spotted the crew of the Iran Ajr dropping what the Americans identified as mines near an anchorage in international waters about 50 miles northeast of Bahrain at about 2 a.m. Persian Gulf time Monday, according to Defense Department officials.

The Iranians had dropped one or two mines overboard before U.S. helicopters began firing at the ship, Rear Adm. Harold J. Bernsen, U.S. Middle East Force commander, told reporters in a Pentagon media pool dispatched to the command ship LaSalle yesterday.

The first attack from the helicopters disrupted the operation. But 34 minutes later, the Iranian crew reportedly resumed dropping mines, Bernsen said. He said the U.S. aircraft opened fire again with machine guns and rockets, setting the ship's stern afire. Officials said the Iranian ship did not return the fire. Survivors fled in a motorized lifeboat and an inflatable rubber boat equipped with an outboard motor.

"The sailors left behind three dead and a ship {pocked} by machine-gun fire and stained with blood," according to the pool report filed by journalists after they toured the damaged, 750-ton Iranian ship.

U.S. Navy special operations SEAL teams were the first Americans to board the damaged vessel almost seven hours after the attack, according to Pentagon sources. U.S. naval forces waited until daybreak to approach the crippled ship and begin rescue operations because they believed the Iranians had planted six or seven of the floating mines before the vessel was disabled, Defense Department sources said.

In addition to the three dead crewmen, the boarding party found 10 mines, fuses and mine-laying equipment, according to a Pentagon spokesman. The mines were similar to the spiked contact mine that damaged the reflagged Kuwaiti tanker Bridgeton during the first U.S. escorting mission through the gulf July 24 and prompted a major buildup of mine-hunting military craft, Pentagon officials said.

"Windows in the bridge were shattered, rooms ransacked and personal belongings strewn about the living quarters," according to the pool report. Military sources told the reporters "there was evidence to indicate that documents aboard the ship had been destroyed in the lower decks before the crew abandoned it."

About two hours after the SEAL -- sea-air-land -- team boarded the Iranian ship using a small patrol boat, U.S. naval forces found a raft with 10 survivors floating in the gulf. The Americans spotted one floating mine, and moving with great caution to avoid any others, the rescue teams plucked an additional 16 Iranian sailors from the water, according to Defense Department officials.

Military officials obtained some information from an English-speaking officer rescued from the ship, Pentagon sources said.

The officer, who had a shrapnel wound in the jaw, told reporters he was pleased with his treatment by U.S. sailors. Reporters said the Iran Ajr's captain, whose right hand was bandaged, also was held on the LaSalle.

The Iranian sailors were guarded in a vehicle storage room of the LaSalle by eight U.S. sailors carrying M14 assault rifles and shotguns. Some of the captives were lying in rows of cots, their hands and legs bound.

Bernsen said the ship had been under surveillance, but declined to say whether that had begun before or after it left the Iranian port of Bandar Abbas. He said the ship was tracked by radar before being seen visually, and mines were spotted when the crew pulled back tarpaulins on deck. He dismissed as "ludicrous" Tehran's claims that the ship was carrying food, not mines.

"There had seemed to many to have been a decrease in the demonstrated hostility of the Iranians toward us," Bernsen told reporters on the LaSalle. "But it's obvious that terrorist acts were still on their mind, that planning was going ahead for things such as this mining incident."

Bernsen added, "Under the present circumstances in the gulf, you can't be complacent. You can't let down your guard . . . . I'm just happy that in this case we were able to catch them in the act . . . . Perhaps they were a bit bolder than they should have been."

White House spokesman Fitzwater said yesterday that U.S. officals "have photographs and film" of the mine-laying operations and that the administration plans to "place that evidence before the United Nations," which is considering a cease-fire resolution between Iran and Iraq in their eight-year-old war.

The U.S. Army helicopter special operations crews -- trained in night observation techniques -- "were very sure of what they saw," said one Pentagon official.

Bernsen said, "There weren't any warning shots in this engagement. These guys were performing a hostile act, putting mines in the water. Warnings don't make sense at that point in time. You want to stop them and we did."

The State Department yesterday went to great lengths to establish a legal basis under international law for the U.S. military attack on the Iranian ship.

Department spokeswoman Phyllis E. Oakley noted that under the U.N. charter every state had the right to use "reasonable and proportionate force" in self-defense, "including the defense of its warships and other vessels flying its flag against actual or imminent attack."

Where mines were laid in international waters threatening U.S. ships "we have the right to respond with the force necessary to terminate that threat. This was the case in the current incident," she said.

Oakley said it was "speculation" whether there was any basis in international law for the United States to put those aboard the Iranian landing craft on trial in an American, or international, court for planting the mines.

On Sept. 13, the United States apprehended in international waters off Lebanon a Lebanese Shiite extremist, Fawaz Younis, and brought him here for trial in an American court under antiterrorist legislation covering anyone involved in taking an American hostage abroad.

Staff writers David Hoffman, Helen Dewar and David B. Ottaway contributed to this report.