A battered truck pulls up to the Israeli lines carrying 12-persons - refugees trying to go back to what's left of their homes in southern Lebanon.

On orders from Israeli soldiers, they climb down. All their belongings, pathetic bundles of clothes and bedding, are opened and searched. Their papers are examined, and they pass through.

Around them, on both sides of the road that links this village with Tyre, 10 miles west, Israeli bulldozers and graders are grinding away at the rocky soil. They have thrown up massive earthworks and fortifications, stretching over more than a mile, to protect their tanks and trucks.

The tanks, some flying red-and-white skull-and-crossbones flags, are dug into deep emplacements for what Western military observes say is apparently going to be a long stay.

A small group of journalists watches the bulldozers, the tanks and the search of the refugees from a shattered house about 50 yards away.

The house was abondoned when the Israelis came, the refrigerator still plugged in and flowers still in the vases. There is a gaping shell hole in one wall.

An Israeli officer approaches. "You tell those people over there," he says, pointing to the detachment of French U.N. troops who allowed the reporters to pass their checkpoint, "that we don't want anybody going into that house. Our guns are trained on it because we are afraid terrorists will come through the woods and if anything happens we will shoot there first."

The French themselves cannot cross to the Israeli side of the line except by prearrangement. Palestinian soldiers in other parts of Lebanon have been seen wearing helmets and caps of U.N. blue, so the Israelis, fearing infiltration, demand names and vehicles numbers in advance before permitting U.N. troops to cross their lines from the Lebanese side, the French say.

This uneasy scene is a reflection of the situation all along the confrontation line in southern Lebanon more than two weeks after the Israeli invasion. Refugees who prefer living under Israeli rule to staying in camps, are trickling homeward.

Only sporadic rifle bursts and rockets disrupt the cease-fire. The tentative, hesitant deployment of the U.N. peacekeeping force continues, though there is no sign that the Israelis whom they are supposed to supplant are planning to leave. Everywhere, from the Hasbani River in the east to Tyre on the coast, there is the potential for a new round of violence.

As with the Lebanese civil war of 1975-76, the Israeli invasion of southern Lebanon has failed to settle the issues. So when it stopped, it left armed forces with conflicting and sometimes uncertain interests facing each other in volatile proximity.

Well-armed Palestinian guerrillas still control one of the three bridges over the Litani River and the Israelis want them removed, as well as several towns and armed strongholds north of the river from which they can easily fire into Israeli territory.

At their mountaintop fortress in the medieval crusader castle of Beaufort, which commands much of the Litani valley and overlooks Lebanese villages held by the Israelis and their Lebanese Christian allies, Palestinians who have been reinforced by "volunteers" from Iraq say they will continue to fight.

A Palestinian field commander there told visitors yesterday that "we don't have any orders from out political leadership" to stop harassing operations behind Israeli lines, even though PLO Chairman Yasser Arafat has agreed to a case-fire along the front lines.

The Israelis might be able to bomb the Palestinians out of Beaufort, though it seems unlikely. And they might be able to drive them off the Litani bridge north of Tyre and out of Nabatiyeh by a renewal of their armored advance. Failing that, there is no force on the horizon capable of doing it, so the conflict is likely to continue.

The deployment of a 4,000-man U.N. force, now about a quater complete, was intended to defuse this time bomb, but the question has not been answered of what the U.N. troops are really supposed to do - keep the Palestinians out of the south, or try to take over Beaufort Castle, Tyre and Nabatiyeh, or forget the area north of the river and just replace the Israelis.

The U.N. commander, Maj. Gen. Emmanuel Erskine of Ghana, acknowledged in an interview with Israel radio yesterday that he did not know what the mandate of his forces was. But whatever it is, he said, they are prepared to shoot if necessary to enforce it.

Unarmed U.N. observers have been harassed and abused in the south for years, he said, and "this is going to stop. We have the weapons. We have the mandate to fire back and the soldiers will do it. They have the instructions to fire back. But he did not say at whom.

The French, who are taking the unusual step of bringing in light tanks for their forces in the south, say they have been ordered to fire on the Israelis if the Isralis try to advance any farther into Lebanon.

The prospect of the United Nations fighting the Israelis, either to block a new advance or to chase them out of the south, seems highly unlikely to observes here. But so does any other possible use of armed force by the United Nations - to dislodge the Palestineians from their strongholds.

As Erskine admitted, there are armed forces on both sides of the Litani - Israelis, Lebanese Christian militias, Moslem dissidents from the Lebanese army and Palestinians - who are simply beyond U.N. control.

The Israeli entrenching operation here is being duplicated at other points, according to correspondents who have visited other sites. This reinforces the impression that the Israelis expect to keep some of their units at key river crossings and road checkpoints ever after full U.N. deployment.

Military analysts in Beirut believe that even if the Israelis eventually retreat toward their frontier, they will insist on keeping certain hilltops and crossroads at strategic points.

Now that they have waded into the Lebanese quagmire, the Isrelis, like the Syrians before them, are apprently discovering how difficult it is to extriacate themselves from it - assuming that they want to, which the Palestinians and many Lebanese question.

While they stay, they form another unpredictable element in the murky mixture of Palestinians, Lebanese factions, innocent civilians and conflicting political and strategic interests that is always threatening to boil over.