A rocket explosion here this morning in which an Israeli soldier was slightly wounded failed to prevent the scheduled session on the withdrawal of Israeli troops from Lebanon between U.S., Israeli and Lebanese negotiators.

Israeli military spokesmen, whose zone of occupation begins just north of this suburban Beirut coastal area, said the single round fired was a Soviet-made 107-mm Katyusha rocket frequently used by the Palestine Liberation Organization in Lebanon.

Israeli spokesmen said they were able to identify the shell that was fired as the Soviet-made Katyusha by its tail. They brought the twisted spent round into the press room of the war-scarred beachfront hotel, where negotiators were meeting, for journalists to see.

They said they believed it was fired from a location about four miles northeast of here near where U.S. Marines are deployed on the edges of Beirut International Airport. Marine spokesmen said they considered it extremely unlikely that it could have been fired from their area; they said they neither heard nor saw any outgoing round and detected no such suspicious activity in their zone this morning.

The rocket fell in front of an eight-story, buff-colored apartment building. It spewed metal and dirt in a muffled explosion that was audible from the hotel about 450 yards away where negotiators had gathered only minutes earlier.

Second-story windows of the apartment building were shattered in the explosion, which frightened families living there. The only report of injury was that an Israeli soldier at a checkpoint alongside the apartment building was hit on an ankle by either a rock or shard of shrapnel.

The negotiators themselves appeared unruffled by the blast when journalists were ushered in about 10 minutes after the explosion for a brief photo session.

Later, Israeli Foreign Ministry spokesman Avi Pazner said, "We feel very secure and we will not let terrorism interfere with our talks."

However, the incident did appear to revive tensions between U.S. Marines and Israeli Defense Forces whose zones adjoin one another.

Marines have scrupulously avoided any contact with Israeli troops, concerned that there be no impression that the two forces are working in coordination. Israeli soldiers, who have been the target of car bombs and snipings, have suggested that some of the attacks are coming from areas patrolled by the Marines.

The PLO claimed responsibility Monday for a Jan. 7 attack on an Israeli troop bus south of Beirut. A military spokesman quoted by the Palestinian news agency Wafa said two guerrillas had been killed in the operation in addition to 40 Israeli soldiers killed or wounded. Israel at the time said that 18 soldiers had been wounded.

On several occasions during the past two weeks Israeli troops have approached Marine positions, drawing strong protest from American officials concerned that the probing could eventually erupt into a clash between the two forces.

In Washington, the Pentagon denied a report in Monday's Washington Post that a U.S. Marine in the multinational force in Lebanon had an Israeli soldier in his rifle sights and had to be ordered not to fire.

Privately, Americans here were skeptical about the Israeli military spokesmen's professed certainty about the area from which the rocket had been fired. Shortly after the explosion Israeli riflemen, backed by an armored personnel carrier, combed the beachfront south of the hotel. The Israeli soldiers in that area rounded up several persons, briefly detaining this reporter and about eight Lebanese bystanders he had been interviewing.

By all indications, the withdrawal talks here made no progress, stalled by the new Israeli demand that some of their soldiers be allowed to remain in Lebanon to man security installations. Reports of Israeli and Lebanese spokesmen indicated that much time was taken by long speeches from both sides reviewing the history of their relations.

Negotiators reportedly were unable to agree on the date or the location of the next meetings before the session ended late this afternoon.