Israeli soldiers occupying new forward positions in a southern suburb of West Beirut harassed Lebanese Moslem militiamen with sniper fire today despite assurances given to the Lebanese government that they were withdrawing.

The Israeli troops, estimated to number from 40 to 50 infantrymen, occupied the Bir Hasan neighborhood adjacent to the Kuwaiti Embassy yesterday in an apparent breach of the Aug. 12 "cease-fire in place" that was one of the cornerstones of the U.S.-mediated accord that paved the way for the PLO evacuation of Beirut.

The Israeli advance, initially explained by Israeli military officials in East Beirut as a screening move to protect mine-clearing operations in the neighborhood, has alarmed West Beirut's Moslem and militia leaders who have expressed fears that the operation may be a prelude to an Israeli advance into nearby Palestinian refugee camps.

After Prime Minister Shafiq Wazzan complained of the Israeli advance yesterday to U.S. Ambassador Robert Dillon, Israeli officials passed word to him that they would withdraw to their previous positions about 700 yards south of Bir Hasan by this morning.

The Israelis, however, did not pull back today despite government radio reports that they had done so.

The Israelis did stay off the shell-pitted four-lane boulevard leading from the Mediterranean seacoast past the Kuwaiti Embassy where they had set up roadblocks and machine-gun posts yesterday. But they remained very much in evidence in the warren of bombed buildings immediately to the south of the road.

This afternoon, the Israelis, who had been seen moving around inside Bir Hasan most of the day, opened fire on a leftist militia roadblock. There were no injuries among the militamen.

The Lebanese government, and Israel radio, did announce today that the Lebanese Army had been allowed to install itself in the Henri Chehab barracks, south of Bir Hasan, in accordance with the government's "security plan" to move Army troops into their former barracks as a reserve for national police, who are supposed to reimpose government authority in West Beirut.

The Army arrival at the southern barracks, which they had abandoned years ago because of fear of attack by the PLO, does not put them between the Israelis and the city, however. The Chehab barracks remain behind Israeli lines and thus do not block any move the Israelis might make.

In an act that appeared to be aimed at appeasing the Israelis and showing the PLO compliance with the letter of the Habib accords, the PLO's remaining diplomatic representative in Beirut, Shafiq Hout, today presided over the handover to the Army of some PLO ammunition stores and some pieces of heavy weaponry that had not been turned over to the militias.

The move was more symbolic than significant. Before the PLO completed its evacuation from Beirut earlier this week, they unloaded other huge stocks of ammunition and heavy weapons to the various militia groups who had fought at their side against the Israelis.

Meanwhile, Abu Iyad, the PLO security chief, was reported back in Lebanon just three days after he had left on the last shipload of PLO fighters.

Abu Iyad was reported by official Lebanese sources to have reappeared in the northern Lebanese port of Tripoli to inspect PLO troops, said to number about 2,000, based there. Israel has maintained that it will not withdraw from Lebanon until remaining PLO and Syrian Army forces in Tripoli and the Bekaa Valley east of Beirut are withdrawn also.