MANAMA, Bahrain (AP) _ An Iranian helicopter gunship fired a rocket into a Cypriot tanker Thursday that set the crew's quarters afire, killing at least one man and injuring six, marine salvage executives reported.

They said three crew members were missing and feared dead. Tugboats were reported still fighting the blaze late Thursday night.

Salvage executives, who spoke on condition of anonymity, said the 30,000- ton tanker Wise was hit Thursday morning in the central sector of the Persian Gulf while bound from Bahrain to Australia with a full load of gasoline.

One said the fire was limited to the stern section, away from the gasoline tanks.

The latest commercial-shipping victim of the long Iran-Iraq war was attacked in the narrow Shah Ullam shoals 70 miles northeast of Qatar, where Iranian aircraft have attacked three other tankers since Sunday.

Iran's attacks apparently are in response to Iraqi air raids since Feb. 26 that crippled four chartered tankers ferrying crude oil from the huge Iranian oil export terminal at Kharg Island to a makeshift facility at Sirri Island, which is 350 miles south of Kharg and beyond the range of Iraqi warplanes.

In the land war, Iran said its forces killed 400 Iraqi soldiers in an overnight attack on two infantry brigades near the southern oil port of Faw.

Tehran radio, monitored in Nicosia, Cyprus, said 800 Iraqi soldiers were wounded in Faw, and Iranian assault troops ''completely crushed'' two brigades and a tank battalion.

The official Iraqi news agency, also monitored in Nicosia, made a conflicting claim. At 3 a.m. Thursday, it said, Iraqi forces overran key Iranian fortifications on the Faw peninsula, enabling Iraqi units ''to control major defensive positions and barriers.''

Iran captured Faw and a large area of the peninsula in an offensive it launched Feb. 9 across the Shatt al-Arab waterway that divides the two nations, which have been at war since September 1980.

Iran sent another invasion force across the mountains of Kurdistan in northeastern Iraq on Feb. 24.