MANAMA, Bahrain (AP) _ Iraq said Thursday that Iran launched a major two-pronged offensive in the southern part of their warfront, but that Iraqi forces had ''wiped out'' its leading edge and blunted the attack.

State-run Baghdad Radio and television interrupted regular programs shortly after midnight to alert the nation to an imminent announcement.

Minutes later the radio, monitored in Bahrain, quoted a military spokesman as saying that the leading Iranian units had been ''wiped out with concentrated firepower'' by the defenders.

The spokesman said the offensive was ''bogged down'' by resistance from the third and seventh army corps. ''The vanguard units have been annihilated, while others were isolated from the rest of the attacking forces by Iraqi fire,'' said the spokesman.

There was no immediate comment on the reports from Iran. It could not immediately be determined whether the attack was the long-heralded ''final offensive'' Iran has promised before the Persian New Year, which begins in March.

Iran reportedly has massed more than 650,000 troops along the war front for an offensive it says will end the war that began in September 1980. About 400,000 of those soldiers were believed to have been concentrated in the south.

Arab diplomatic sources say Iran apparently counted on cloudy and rainy weather to neutralize Iraq's superior air power, but Iraq has stepped up air strikes behind Iranian lines to forestall the offensive.

Iraq also says it has mustered a half-million soldiers and volunteers to confront the Iranian attack.

Iraq's third army corps defends an area east of the southern Iraqi port of Basra near the Shatt Al-Arab estuary. The seventh army corps guards the southernmost regions of the 700-mile war front, touching Iraq's Faw peninsula that Iran occupied last February.

Iraq said the offensive begasn about 9:10 p.m. (1:10 p.m. EST) Wednesday. The Iraqi spokesman indicated the fighting still was raging at 1:00 a.m. Thursday.

Baghdad Radio blared martial music and announcers assured the ''people of Iraq and the rest of the Arabs everywhere ... the forces of evil (Iranians) have come and their advance units have so far been destroyed, annihilated and isolated from the rest of the aggressor forces.''

''The Iranians have perpetrated their offensive in an attempt to camouflage their dealings with Zionism and imperialism,'' the spokesman said, in an obvious allusion to recent secret U.S. arms shipments to Iran via Israel.

Basra was the scene of three major offensives in which Iran used ''human wave'' tactics in futile attempts to seize southern Iraq. The Iranians last attacked the Basra region in March 1985, when an estimated 20,000 of its soldiers were killed.

Earlier, Iraq said its air force struck at Iranian troops and industrial targets in 15 raids Wednesday, the third straight day of sustained Iraqi air attacks.

Iraq's air force commander, Lt. Gen. Hamid Shaaban, vowed his jets would keep hitting Iran's economic lifelines and added that ''no Iranian will be safe from our burning fire.''

Baghdad radio, monitored in Nicosia, said Iraqi jets hit troop concentrations and transport hubs in Sumar and Shushtar in western Iran and Dezful farther south; the oil refinery and power station in the western city of Kermanshah, also known as Bakhtaran, and a Hawk anti-aircraft missile battery.

Iran says at least 275 civilians have died since Sunday in Iraqi air raids on Bakhtaran and the nearby city of Islamabad-e-Gharb.

Iran said Iraqi jets bombed an Arab-owned cargo ship, the Ibn al-Beitar, impounded by Iran and moored at an unspecified Iranian port.

Tehran's Islamic Republic News Agency, also monitored in Nicosia, said two Iranian coast guardsmen were seriously wounded in the attack.

The ship's bridge and engine room were heavily damaged and the 23,618-ton freighter was about to sink because its hull was holed, the agency said.

It did not say when the attack took place.

The ship is owned by the Kuwait-based United Arab Shipping Co., which in turn is owned by Kuwait, Iraq, Saudi Arabia and other Persian Gulf Arab countries.

Iran intercepted the ship on Nov. 24, 1985, and confiscated it, claiming it was carrying Iraq-bound war supplies.