The U.S. helicopter fired upon by Iranian boats in the Persian Gulf last week was a heavily armed Army gunship, not an observation aircraft as initially reported by the Pentagon, according to Defense Department sources.

Sources said the helicopter, an Army special operations McDonnell Douglas MH6 craft armed with machine guns, rockets and high-technology night-vision equipment, was flying with two similar Army copters on a mission in search of mine-layers and attack boats about 15 miles southwest of Farsi Island.

When the crew of the first helicopter spotted tracer rounds from gunboats in the darkness below, it alerted the two sister gunships, which swooped in and began firing at the vessels without any warning, sources said.

Pentagon sources said the entire exchange lasted only a few minutes.

The new version of the attack comes amid congressional concern over the United States' steadily expanding military role in the war-torn Persian Gulf.

Defense Department officials originally said the Iranian gunboats fired at a patrol helicopter, which radioed for help, and that two other military helicopters then flew in to assist the first.

Defense Department officials "stand by" the initial version of the incident, a Pentagon spokesman said yesterday.

Defense Secretary Caspar W. Weinberger, in a speech Friday, said, "We had three helicopters up and they were fired on," making no reference to an observation helicopter. In the same speech, Weinberger noted, "We always beware of initial reports. They're hardly ever totally right."

The three U.S. helicopters sank a Swedish-made Boghammer patrol boat and disabled two Boston Whaler-type boats while a Corvette speedboat escaped, Pentagon officials said. Six Iranian crew members were rescued after the attack; two died of their wounds. Tehran has said six others are missing.

Weinberger in his speech said the helicopter crews fired at the Iranian boats without seeking permission from higher military authorities.

"Under those circumstances with our rules of engagement {they} are not required to seek committee approval," Weinberger said. "When you have a clear hostile act -- I can't imagine anything more hostile than being shot at -- you have the authority immediately, automatically, to respond appropriately."

Iranian officials have denied that their boat crews fired the first shots. But some observers in the region suggested the Iranian crews may have considered themselves in danger when the helicopters approached and may have fired in self-defense. A few days before the incident, Farsi Island had been bombed by Iraqi warplanes. Defense Department sources said the U.S. helicopter attack took place three miles outside the 12-mile wide Iranian territorial zone surrounding the island.

Pentagon officials said there was no radio communication between the Iranian boats and the U.S. aircraft prior to the attack and no indication the Iranians knew the helicopters were American. Using night-vision equipment, the crews of the U.S. helicopters were certain the boats were Iranian, Pentagon officials have said.

Weinberger said the three U.S. helicopters were on "normal patrol" when the first shots were fired in the darkness about 9:50 p.m. gulf time Thursday. Weinberger said the aircraft were looking for mine-layers and concentrations of Iranian gunboats.

Military officials said the Army helicopter teams have increased their nighttime surveillance missions in recent weeks because of a buildup of Iranian gunboats in the northern Persian Gulf.

The four surviving Iranians are being treated aboard the transport ship USS Raleigh while awaiting return to Iran through international diplomatic channels.

No U.S. military personnel were injured and no helicopters were hit by Iranian gunfire, officials said.

The Navy yesterday released four photographs that it said showed the two Boston Whaler-type Iranian boats on the deck of an unidentified U.S. warship, according to a media pool report aboard the USS LaSalle, the command ship for American forces in the gulf.

U.S. military officials refused to allow journalists to see, photograph, or view videotape of the two boats, according to a report filed by the media group.

The journalists were denied direct access to the boats because of "operational security," according to Lt. Col. John Head, a spokesman for the U.S. Central Command, which controls gulf operations for the American military.

Military officials allowed a pool of journalists to board the minelayer Iran Ajr after it was attacked and captured Sept. 21. Defense Department officials used the journalists' photographs and news coverage of the mines on board the ship to refute Iranian claims that the vessel was carrying food.

The photos released by the Navy yesterday showed a Soviet-model 12.7 millimeter machine gun mounted in the bow of one of the vessels captured last week. There was no weapon visible in the other craft. The fiberglass hull of one boat appeared to have been shredded by machine-gun fire.

Weinberger said Friday that U.S military officials found shoulder-fired Stinger missile equipment onboard. Pentagon officials said batteries and packaging for the surface-to-air missiles was discovered on at least one of the boats. The photographs showed none of that material.