The Israeli Army mounted a large-scale airborne and mechanized infantry operation in southern Lebanon today after an ambush by Moslem militiamen in which two Israeli soldiers were captured and two pro-Israeli Lebanese militiamen were killed, the Army command announced.

The attack and search operation about seven miles north of the border, inside the Israeli-declared "security zone" patrolled by Israeli troops and militiamen of the allied South Lebanon Army, failed to turn up any sign of the two captured soldiers and an SLA militiaman who were seen being driven away in a black sedan, Israeli military sources said.

The Islamic Resistance Movement said in Lebanon that it was holding the two Israeli soldiers and treating them for "serious wounds" suffered in the ambush, but it did not say where they were being held, Washington Post special correspondent Nora Boustany reported from Beirut.

The group, which has claimed involvement in past anti-Israeli attacks, said it would soon release a photograph of the Israeli captives and announce a list of demands for their release. It also claimed that 10 Israeli soldiers had been killed in the ambush.

It was the first capture of Israeli soldiers in southern Lebanon since Israel withdrew most of its forces from the area last June. Three other Israeli soldiers have been in captivity -- in Damascus, according to intelligence sources here -- since the time of the Israeli invasion of Lebanon in 1982.

A spokesman for the U.N. peace-keeping force in Lebanon said tonight that three columns of Israeli mechanized infantry and helicopter-borne troops entered several villages in the area of Kounine, northeast of the town of Bint Jebil, and searched until nightfall.

Timur Goksel, the U.N. spokesman, said by telephone from Naqoura, the U.N. headquarters in southern Lebanon, that by 7 p.m. the Israeli forces had regrouped in the village of Shaqra for the night, having searched the villages of Taire, Bayt Yahoun and Sultama. Goksel said there were no confrontations between Israeli forces and the U.N. peace-keepers, whose area of operation begins about 1 1/2 miles north of Kounine.

Details of the ambush remained sketchy late tonight, but security sources said two vehicles in a three-vehicle patrol were riddled by machine-gun bullets as they drove along a road near Kounine.

Israeli military sources said the vehicles were headed north from Bayt Yahoun about 12:30 p.m., the first carrying only South Lebanon Army militiamen, the second two South Lebanon Army soldiers and an Israeli "education officer," and the third two Israeli soldiers and an SLA driver.

The sources said the first car passed the ambush without being hit, but that the second was rocked by a land mine that killed two South Lebanon Army militiamen. The Israeli escaped unharmed.

The third car was fired upon, and the two Israeli occupants were captured and taken away, Army sources said.

The Army command said Israeli troops were quickly brought in by armored vehicles, helicopters and gunships in the Mediterranean.

Several Moslem militias in southern Lebanon said they had carried out the ambush, according to Army sources in Tel Aviv.

On Dec. 30, two South Lebanon Army militiamen were killed and an Israeli soldier was wounded in an ambush near Kounine that prompted a similar operation by Israeli troops. Some houses in which the ambushers were said to have been harbored were blown up by the Israeli force.

Israeli officials refuse to say how many troops they have in southern Lebanon in the narrow, irregularly shaped security zone that stretches from the Mediterranean Sea to the foothills of Mount Hermon. They say only that they maintain liaison units at the South Lebanon Army headquarters in Marjuyun, and occasionally make "search and arrest" forays to counter Moslem militias operating in the area. However, security sources in southern Lebanon have said that Israel maintains a fully mechanized battalion of 400 men near Hasbayya, north of the U.N. zone; a mechanized infantry company of 100 men north of the Crusader-era Beaufort Castle, and, in the Finnish U.N. battalion's area, a company of 100 troops that is equipped with Merkava heavy tanks with night-vision apparatus.