NICOSIA, Cyprus (AP) _ Iran claimed Tuesday that marines led by frogmen seized an Iraqi radar platform in the Persian Gulf that directed air raids on oil tankers.

Iran also claimed its forces killed 700 Iraqis in a northern offensive, but Iraq declared that its forces repulsed the Iranian drive in the mountainous Haj Omran sector at the northern tip of the 730-mile-long border warfront.

Iraq, in a statement issued late Tuesday, claimed 3,000 Iranian soldiers were killed when the offensive was crushed on the northern warfront. Iraq did not elaborate.

But the failure of Iraq's military spokesmen to claim the Iranians were driven off the gulf with heavy casualties, which they customarily do, indicated the radar platform had been taken.

The later Iraqi statement claimed only that its jets sank 32 Iranian boats and killed an undetermined number of crewmen off the al-Amiq radar platform. It also said the Iraqis conducted 176 air attacks on the north, inflicting heavy losses on Iranian troops, and that one Iraqi warplane was shot down.

''We hold the Iranian side responsible for the safety of its pilot,'' said the statement, which was monitored in Nicosia.

Conflicting claims cannot be reconciled because neither side allows foreign journalists into battle areas except on rare guided tours.

The two-pronged Iranian thrust could be designed to stretch Iraq's defenses before Iran's long-heralded ''final offensive'' to end the 6-year-old war.

Military analysts believe it will be concentrated in the desert flatlands of the central and southern sectors and will involve an effort to cut off Baghdad, the Iraqi capital.

Accompanying the new assaults is a huge Iranian mobilization that some accounts say includes more than 800,000 regular troops, fanatical revolutionary guards and ''basij,'' or volunteers.

Most of the force is believed to be opposite Basra, Iraq's second-largest city. The provincial capital is on the western bank of the disputed Shatt al- Arab, which forms the southern border.

The war began in September 1980 when Iraq sent an invasion force across the waterway, its only outlet to the sea.

Iran's official Islamic Republic News Agency said Al-Amiq platform, a converted oil loading facility 25 miles south of Iraq's Faw Peninsula, was seized in a pre-dawn assault by three brigades of revolutionary guard marines led by commando frogmen.

It said all defenders, members of Iraq's 444th Naval Battalion, were either killed or captured.

The Iranian news agency said an Iraqi fighter-bomber was shot down by anti- aircraft fire and a Soviet-made attack vessel was sunk during the ''unprecedented, lightning attack'' on the platform, which covers half a square mile.

Iranian warships and coastal artillery later hammered nearby Al-Bakr platform, ''inflicting heavy casualties and damage'' and leaving it ''engulfed in flames,'' IRNA said.

Coastal artillery pounded the southern Iraqi port of Umm Qasr opposite Faw, the agency said. About 40,000 Iranians have been dug in on the peninsula since storming across the Shatt al-Arab in February.

IRNA claimed seizure of Al-Amiq platform gave Iran surface and air control in the northern Persian Gulf.

Iraq pumped one million barrels of oil a day through the platform before the war began. It was transformed into a missile-defended radar base that directed air strikes against tankers carrying the oil exports with which Iran finances its war effort.

Baghdad radio quoted a military spokesman as claiming Al-Amiq was deserted at the time of the attack but Iraqi forces were ''confronting the enemy with courage.''

Tehran radio broadcast a military communique that said Iranian forces carried out ''an infiltration operation'' in the northern Haj Omran sector during the night and destroyed three Iraqi bases. It claimed 700 Iraqis had been killed in the previous 24 hours.

Iran said Monday that it launched a drive called Karbala-2 in Haj Omran before dawn and captured seven strategic heights in the Kurdistan mountains. The offensive involved an estimated three divisions of Iranian regulars and revolutionary guards totaling about 30,000 men.

By Iranian count, more than 3,000 men of Iraq's 5th Army Corps have been killed and 277 captured in the north.

Tehran's communiques made no mention of Iranian losses. Iraq says it has inflicted ''thousands'' of casualties in the northern fighting.

Baghdad radio claimed the Iranian thrusts in Haj Omran ''failed abjectly'' and enemy soldiers were ''retreating in humiliation, leaving hundreds of casualties.''