MANAMA, Bahrain (AP) _ Iraq said its ground forces recaptured part of the Majnoon Islands in a ''heroic thrust'' on Iranian strongholds Monday. Iran said the Iraqi offensive on the oil-rich region was repelled.

Iraqi President Saddam Hussein, in a speech on the 65th anniversary of the creation of his nation's armed forces, warned Iranian leaders of a ''major catastrophe'' if Iraqi territory is invaded. He promised to turn the southern Huwaizah marshland into a ''mass grave for the aggressors,'' Baghdad radio reported.

An Iranian military communique, carried by Iran's official Islamic Republic News Agency and monitored in Cyprus, said Iraqi attackers suffered hundreds of casualties ''in this desperate attempt'' to retake the islands in the 926- square-mile Huwaizah marshes.

''Iranian forces continued to defend their positions and are in full control of the operation region,'' the Iranian communique said.

The military activity could not be independently verified. Reporters and neutral observers are seldom allowed to visit the front line.

The two countries have been engaged in a bloody and inconclusive border war on the northern flank of the Persian Gulf since September 1980.

The Baghdad high command reported a pre-dawn, two-pronged offensive into Iranian-occupied positions across Majnoon, a chain of three small islands on the southeastern tip of the Huwaizah marshes.

Iranian forces lost ''hundreds of fatalities, many more of injuries and large numbers of prisoners,'' according to an Iraqi communique broadcast on Baghdad radio and monitored across the gulf in Bahrain.

Earlier, Iraq announced that ''a shattering offensive'' launched during the night had liberated the islands.

Iranian forces overran Majnoon in February 1984. An Iraqi claim last year to have regained parts of the Majnoon oil fields was denied by the Iranians.

Bracing for an anticipated offensive by 500,000 Iranian fighters, Iraq has built dikes and set up concrete barriers across the marshland.

On other positions along the front line, the Iraqi communique said Iraqi jets staged 143 bombing raids on Iranian, including one that ''dealt a blow'' on Iran's main Kharg island oil terminal.

Iraq has been claiming to have staged daily air raids against Iranian troop concentrations along the 733-mile-long front line.

Hussein, 48, vowed to pursue more military attacks on Kharg island as well as other Iranian targets for the purpose of ''curtailing the enemy's ability to export oil.''

Iraq has called for a mediated settlement to the war, but Iran insists that before peace can be discussed, the Iraqi president must be overthrown and tried as a war criminal.