U.S. Navy helicopters attacked and set on fire an Iranian Navy amphibious vessel reportedly laying mines in the Persian Gulf last night only hours after an Iranian gunboat attacked an oil tanker flying the British flag, setting that ship ablaze and leaving two crewmen missing.

The two attacks marked another escalation in the fighting in the region and came a day before Iranian President Ali Khamenei is scheduled to address the United Nations General Assembly in New York.

U.S. helicopter crews, using infrared night-vision sensors, spotted "objects being dropped over the side" of an Iranian Navy amphibious landing ship near an anchorage in international waters about 50 miles northeast of Bahrain, according to Pentagon sources.

Pentagon spokesman Fred S. Hoffman said the helicopter crews identified the objects as mines and reported the incident to the commander of the U.S. Middle East Force, who gave the crews permission to fire on the Iranian vessel. The helicopters fired 7.62-mm machine guns and 2.75-inch air-to-surface rockets, setting the stern of the ship, the Iran Ajr afire, Hoffman said.

"The U.S. actions were purely defensive," Hoffman said. "Planting mines in international waters is a violation of international law and an unjustified threat to shipping of involved nations."

U.S. military personnel "had been following this ship, suspecting it was laying mines," said one Pentagon source. "We saw them doing it and they were targeted." The ship's crew began laying the mines after dark, sources said.

The fire on the Iranian vessel appeared to have been extinquished and U.S. ships and aircraft were standing by to provide assistance last night, said Hoffman. He added it was uncertain whether there were any casualties.

U.S. officials in the gulf, concerned that some of the mines may have spilled out into the water, were using caution, however, waiting for daylight to approach the ship, according to Pentagon sources.

"Nobody wants to get too close," said one Defense Department source.

The Iranian vessel is a World War II-vintage amphibious landing ship believed to have been sold by the United States to the Iranian regime of Shah Mohammad Reza Pahlavi, according to Pentagon sources. Pentagon sources said the ship is part of the regular Iranian Navy; previously U.S. military officials have expressed worries that the Iranian Revolutionary Guard, the more unpredictable branch of the Iranian military, has laid mines.

The area in which the mines were being laid is an anchorage used extensively by commercial vessels and has been used by Kuwaiti tankers re-registered under the U.S. flag and their Navy escort ships, according to Hoffman.

The earlier attack on the British-flagged ship was the most serious on a vessel registered to a nation with forces in the gulf. An Iranian gunboat fired a "missile" -- probably a rocket-propelled grenade -- into the tanker, the Gentle Breeze, setting it on fire and leaving two crewmen missing, according to gulf shipping sources quoted by Washington Post foreign correspondent Patrick E. Tyler, in Cairo.

The Gentle Breeze, a 102,799-ton crude carrier, radioed a distress call shortly after 9 p.m. local time saying: "Mayday. The attack has now stopped. Two crew members are missing. Require fire-fighting tug." The ship was later reported ablaze off the Saudi Arabian coast.

The shipping sources, reached by telephone, said the gunboat attacked as the Gentle Breeze passed near Farsi Island, which has been used by Iran's Revolutionary Guards to mount naval assaults against merchant shipping.

Although Britain has at least two frigates and a destroyer in the gulf, the Gentle Breeze apparently was steaming at night without warship escort.

British naval vessels have habitually accompanied British-flag merchant ships only as far as Bahrain, well south of where the tanker was hit last night.

British Foreign Secretary Geoffrey Howe, at the United Nations, said the attack on the tanker would not bring a reinforcement of British forces in the region.

"It's one of the urgent reasons for pressing ahead with international action to bring the conflict to an end," he said in an interview on the "MacNeil/Lehrer Newshour."

He said Britain would now concentrate its efforts on pressing for "swift and effective" action by the U.N. Security Council, an apparent reference to a U.S.-backed proposal to bar arms sales to Iran for its refusal to comply with a U.N. cease-fire resolution.

Shipping sources in the gulf, reached by telephone from Cairo, said the Gentle Breeze was steaming empty when the gunboat approached and fired what was described as a missile into the ship's superstructure. Gulf shipping agencies often describe rocket-propelled grenades, a main weapon of the Revolutionary Guards, as "missiles" -- and a Pentagon official said such grenades were used in the attack.

It was not known which part of the tanker was struck. Iranian gunboats typically aim for the crew and machinery areas near the stern.

The American helicopters that attacked the Iranian ship were based on the guided missile frigate USS Jarrett, one of about 30 U.S. Navy ships assigned to the area of the Persian Gulf and Northern Arabian Sea. The helicopters were about 15 miles from the Jarrett when they fired on the Iranian ship, Pentagon spokesman Hoffman said.

The USS Jarrett normally carries two Seahawk LAMPS 3 helicopters. Some Army OH6 Cayuse special operations helicopters are also being used in the gulf operations.

Pentagon officials declined to specify which type of helicopters were involved in the incident. Both are armed with machine guns and air-to-surface rockets, as well as Forward-Looking Infrared (FLIR) sensors for night vision.

It was the second incident in which U.S. military forces fired at an Iranian target since the navy began escorting reflagged Kuwaiti tankers through the Persian Gulf in late July. A Navy F14 Tomcat pilot fired two Sparrow air-to-air missiles at a radar blip believed to be an Iranian F4 fighter on Aug. 10. The missiles missed.

Mines have been a major focus of military forces in the region since the reflagged oil tanker Bridgeton struck a mine during the first U.S. convoy July 24. The United States has dispatched mine-hunting Sea Stallion helicopters, five ocean-going minesweepers and six small coastal minesweepers to the region. The larger ocean-going minesweepers are not scheduled to reach the gulf for several more weeks, according to Pentagon sources.

In the past year, Iran has focused gunboat attacks on merchant vessels bound for Kuwait due to Kuwait's support for Iraq in the seven-year-old gulf war. The attacks prompted the United States earlier this year to protect 11 Kuwaiti tankers -- half of the nation's fleet -- by reregistering them under the American flag.

Of the maritime powers that have sent warships to help protect gulf shipping, only the United States and the Soviet Union have actually placed Kuwait's crude oil exports under their protection either by changing ship registration or through charter arrangements.

Shipping sources in the region had expected an Iranian attack last night after Iraqi warplanes struck late Sunday at an Iranian supertanker, the Khark-2, near Iran's main oil terminal at Kharg Island.

The attacks in the gulf coincide with a diplomatic showdown at the United Nations, where President Reagan yesterday warned Iran to accept a two-month-old U.N. cease-fire resolution or face an arms embargo.

Some U.S. military officials were angry "that the Iranian Navy would have the audacity to be laying mines while the president of their country is here for the U.N. General Assembly," said one Defense Department official.

Meanwhile, Iraq reported that it flew successful bombing missions against Iranian economic targets near Tehran and in northwestern Iran. The Iraqi news agency said its warplanes flew midday sorties against the Ezfeh oil pumping station, which feeds petroleum lines to the Iranian captial, and a power plant and factory near Bakhtaran in the northwest.