The United States has formally expressed its concern to Israel about four recent incidents in which Israeli soldiers approached military positions of U.S. Marines in the multinational peace-keeping force here despite the U.S. policy of confining contacts between the two nations here to diplomatic channels.

The United States, concerned that such encounters could lead to clashes between forces of the two countries, reportedly has asked Israel to avoid them in the future. "They know what the ground rules are," said a U.S. Embassy source here. "It could cause problems."

Marine spokesmen in Lebanon stressed that in none of the four encounters during the past week were the Israeli troops hostile or aggressive and that they left without protest when asked to do so by the Marines.

But Americans here are clearly concerned about the persistence of the contacts. On the one hand, the Marines are concerned about giving any appearance of working in cooperation with the occupying Israeli Army. On the other hand, Americans here worry that if the recent encounters are allowed to continue, they might eventually lead to clashes between Marines and Israeli soldiers.

The potential for such conflict is one of the concerns expressed privately in Washington by top U.S. military officers wary about committing American troops here.

Marines and Israeli soldiers have been operating in adjoining areas of the southern portion of West Beirut since the 1,200-member American contingent arrived here in late September. Until last week, however, the Israelis had not challenged the U.S. policy of confining contacts to diplomatic channels. The Marines and Israeli soldiers were clearly visible to each other but kept to their own areas.

The encounters during the past week came as the Israeli Army began conducting massive sweeps on the fringes of Beirut after an Israeli Army truck was blown up when it passed a booby-trapped limousine parked on a road less than a mile northeast of the Marine area of operations.

According to press reports here, two Israeli soldiers were killed and two others wounded in the explosion. A later investigation indicated that the car contained more than 200 pounds of explosives wired to a detonator in a nearby house.

Shortly after the explosion, Marines were surprised to see Israeli soldiers approaching their position. According to the spokesman for the Marine contingent here, Lt. Col. Walt DeForest, the Marines asked "What are you people doing here? And essentially what they said was, 'We're lost.' " When the Marines asked them to leave, they did, DeForest said.

The first encounter was soon followed by two others and the fourth time yesterday, when the Israelis pulled up to that Marine checkpoint in three jeeps and an armored personnel carrier, according to DeForest.

On that occasion, an Israeli officer asked to see the Marine's commanding officer to inform him that the Israelis would be conducting sweeps adjacent to a Marine position. The Marine commanding officer, Col. Thomas Stokes, declined to see them. The incidents were reported to the American Embassy, which lodged its expression of concern with the Israeli government. The Israeli Defense Force refused to comment.