U.S. helicopters that came under fire when answering a merchant vessel's distress call in the Persian Gulf shot rockets at two suspected Iranian gunboats, American military officials said.

The two small boats Tuesday fired at a pair of U.S. military helicopters that had answered a Panamanian vessel's call for help, the Pentagon said.The helicopters, which were not hit, returned fire with rockets. "It is believed one of the boats was damaged," the Pentagon said.

The exchange came nine days after the USS Vincennes mistakenly shot down an Iranian airliner July 3, killing 290 passengers and crew.

Also Tuesday, Iraq said its troops pushed Iranian soldiers from the Zubaidat border region along the central war front in four hours of fierce hand-to-hand combat, inflicting heavy casualties and capturing large numbers of prisoners.

The Iraqi general command announced the "complete liberation" of the region east of Amara, 190 miles southeast of Baghdad.

Iran made no reference to the Zubaidat fighting. But it admitted its forces staged a "tactical retreat" from Halabja in Iraq's northernmost province of Kurdistan.