QAAQAALIYET AL JISR, Lebanon - On the far side of the Litani River, four white vehicles were clearly visible through the rain. Their arrival yesterday marked another step backward from crisis as they delivered Iranian troops to the Kaitayeh bridge, where the U.N. emergency force in south Lebanon is to set up observation posts.

But here on the north side of the river, where Israeli occupation ends and the fragmentation and chaos of Lebanon resume, there is abundant evidence that full deployment of the 4,000-man U.N. force will take a bit of time. Even when completed it will not eliminate the potential for another explosion.

Palestinian guerrillas of several factions have taken up new positions in a narrow belt that runs almost the width of Lebanon above the Litani. With assistance from Iraq and Syria, they are continuing radom shelling of the Israelis and have introduced a new weapon into their arsenal - the pneumatic jackhammer.

All across the rugged hills of south Lebanon, the Palestinians are digging up the roads and planting mines. Their apparent aim is to hinder any possible Israeli movement further north, but it will also hamper the movement of U.N. troops from this side and impede the return of refugees.

In a tour of the confrontation area yesterday, correspondents found well-armed Palestinian troops - from Yasser Arafat's Fatah organization, the radical Popular Front for the Liberation of Palestine, the Syrian-sponsored Saiqa faction and from the Yarmouk Brigade of the Palestine Liberation Army - pitching tents in the rainy valleys and setting up guns in the empty villages.

The Palestinians have lost their strongholds along the Israeli border and have little maneuvering room to their rear, where they run up against the zone of authority established by Syria after the Lebanese civil war. If reports from Israel that Israel will only withdraw from southern Lebanon when satisfied that there is no further threat from the Palestinians, are correct, however, it may take some time.

As always, the Palestinians in the field tell interviewers that their war on Israel will go on as long as it takes. They claim to be continuing their attacks on the Israeli invasion force despite a cease-fire proclaimed by Israel Tuesday. There was sporadic rocket and artillery fire in the south yesterday.

But the palestinians' say their immediate targets are the U.N. troops that are to be sent into the south from the Lebanese side. In a typical comment, a Yarmuk Brigade commander known as Abu Khalid said yesterday that if any U.N. troops came through his zone, his policy was to "shoot them."

That kind of talk prompted another round of talks yesterday between U.N. and Palestinian officials seeking to ensure a safe passage to the south for those U.N. forces arriving in Beirut. Neither the Palestinians nor the United Nations would release any details of the negotiations, but more than 200 French troops remained at Beirut airport instead of heading south while talks continued.

If arrangements are completed, a French reconnaissance team is scheduled to go on Friday to the port city of Tyre, the Palestinians' only remaining stronghold south of the Litani, informed sources said.

The attention focused on the cease-fire and the arrival of the U.N. force had diverted attention from some difficult questions still unaswered about the mission of that force.

It is unclear, for example, whether the U.N. troops will actually prevent any Palestinians from going back into the zone south of the Litani. Nor is it clear who will have the authority to permit refugees to return there, or what is to become of the Lebanese Christian militias in the zone who have been supported by Israel in the past but now reportedly are firing at the U.N. troops arriving from the Israeli side.

Gen. Emannuel Erskine of Ghana, commander of the U. N. forces in Lebanon, UNIFIL, was in Beirut yesterday to discuss these questions with Lebanese, French and Palestinian officials.

Once all that is sorted out, Lebanon still faces the problem of the refugees whose houses and farms have been destroyed in the fighting - an estimated 200,000, for whom the government has issued an appeal for international, assistance - and of trying to restore the authority of this country over a large slice of its own territory.

Almost forgotten in the uproar over the invasion is that Israel and Lebanon, with the tacit approval of Syria, reached an agreement last fall, by which a rebuilt Lebanese Army was to move into the very zone now occupied by Israel, reassert Lebanese control, and ensure Israeli security against the Palestians. The agreements within Lebanon, and now some new formula will have to be found.