WASHINGTON (AP) _ U.S. military helicopters exchanged fire Tuesday with two suspected Iranian gunboats that were attacking a Panamanian tanker in the Persian Gulf, the Pentagon said.

The boats had been shooting at the 81,282-ton Universal Monarch, which broadcast a distress alert at about 11:30 a.m. EDT saying it ''was being attacked and was on fire,'' said Defense Department spokesman Dan Howard.

''To my knowledge, this is the first such exchange since the incident of (July) the third,'' when the USS Vincennes mistakenly shot down an Iran Air jetliner, killing all 290 people aboard, said Howard.

Two attack helicopters plus a Lamps Mk III surveillance helicopter from the guided missile frigate USS Nicholas were sent to investigate the Mayday call from the tanker, said Lt. Col. David Titus, a spokesman for U.S. Central Command in Tampa, Fla., headquarters for American forces in the gulf.

''As the armed helicopters approached the Universal Monarch, two small boats were seen departing the area,'' said Titus. ''The boats fired at the helicopters at approximately 12 noon EDT and the helicopters returned their fire with rockets and machine gun fire.''

''It is believed one of the boats was damaged. There was no damage to the helicopters and no U.S. casualties,'' he said. ''The Universal Monarch suffered some hull, engine and and fire damage. No casualties were reported and the ship required no further assistance, acording to its master.''

A tug from an unknown country helped the Universal Monarch, Titus said.

It was believed that the attack helicopters were AH-6s, flown by Special Operations Forces attached to the U.S. fleet in the gulf and operated from specially outfitted barges.

The incident took place in the northern Persian Gulf, 23 nautical miles west-northwest of Farsi Island, which has been used as a staging point for Iran's Revolutionary Guards in launching attacks by small boats against neutral shipping in the gulf.

The London-based Lloyd's Shipping Intelligence Unit said the Universal Monarch is owned by a Hong Kong company and managed by the Japanese company Tokudo Shasen K.K., of Tokyo.

The vessel was sailing empty to pick up cargo from the Khafji oilfields, owned jointly by Kuwait and Saudi Arabia, according to shipping executives in the gulf who spoke on condition they not be identified by name.

Since the Vincennes incident, Iran has attacked two commercial vessels - the Norwegian-owned tanker Berge Strand and the Romanian-owned freighter Plataresti.

In the so-called ''tanker war'' - an offshoot of the 8-year-old Iran-Iraq war - Iran attacks neutral shipping in the gulf in retaliation for Iraqi raids on its oil tankers.

Last Thursday, Iraqi warplanes raided two supertankers in the vicinity of Iran's Kharg Island oil terminal at the northeastern end of the gulf. The Greek-owned 264,108-ton Fellowship L and the Cypriot 215,780-ton Star Ray were set afire, but no casualties were reported.

The Iranian ship attacks have continued despite a broadened role for U.S. Navy warships in the gulf, enabling them to protect commercial vessels even if they do not fly the Stars and Stripes.

In another development, the Pentagon announced the completion of the 46th U.S. convoy through the gulf, as the USS Robert G. Bradley finished escorting the reflagged Kuwaiti tanker Townsend to international waters off Kuwait at about 7:40 a.m. EDT Tuesday.

The Bradley and the Nicholas are both Oliver Hazard Perry-class guided missile frigates.