Israeli troops completing their pullout from southern Lebanon today will not turn the positions over to U.N. forces but to rightist Lebanese Christians, a high government official said yesterday.

Such a move could lead to new clashes between the Christian forces and Palestinian guerrillas seeking to reestablish their presence three months after Israel's invasion of southern Lebanon to drive them from the area. The 18-month Lebanese civil war grew out of similar clashes in 1975.

"The Christians will take most of the area we control now," said the official, appearing to confirm the concern raised Saturday by the commanding general of the U.N. peacekeeping force.

Maj. Gen. Emmanuel Erskine, of Ghana, said then that Israel was planning to circumvent the United Nations by turning back the remaining enclaves to the Christian forces that it supports with arms and money.

U.N. officials in New York had said Erskine's remarks were based on a misunderstanding.

Another possible indication of Israel's intention to maintain proxy control in defiance of the United Nations came with the unexpected Israeli announcement yesterday of agreement to reactivate a long-dormant Mixed Armistice Cmmission. The move was seen as an attempt to off-set the expected outcry at Israel's sidestepping of the U.N. assumption of control.

The commisssion, consisting of representatives of Israel, Lebanon and the United Nations, was founded in 1949 but has been largely inactive since 1967. It met yesterday at the U.N. force headquarters in Naqura, Lebanon.

Israel had announced earlier its intention to withdraw today the last of its troops from areas invaded March 14 following a terrorist attack on a tourist bus near Tel Aviv.

It was learned only after Gen. Erskine's critical radio interview on Saturday that the Israelis have already completed partial withdrawals in the phased pullback - including withdrawal from some enclaves retaken by Christian militia.

The reactivation of the armistice commission at such a late hour before the final withdrawal appeared to have more than just symbolic significance. Even though Lebanon and Israeli officials have met from time to time in negotiations, Israel has generally not regarded them as mixed but simply as discussions.

It is known that when U.N. Secretary General Kurt Waldheim visited here in April, Foreign Minister Moshe Dayan said he had been thinking of seeking reactivation of the commission to help iron out problems in south Lebanon.

However, it was pointed out that while Dayan may have had in mind the armistice agreement against cross-border military actions, the commission ostensibly has a responsibility to restrict civilian traffic. Christian Lebanese workers have regularly crossed the "good fence" at the border to go to jobs in Israel since the 1975-76 Lebanese civil war.

Maintenance of the good-fence arrangement was said to be a topic of the meeting in Naqura, along with discussions on infiltration of Palestinian guerrillas south of the Litani River and the question of control of the enclaves.

U.N. officials were known to have concluded that if Lebanon and Israel agreed to keep the fence open to civilian traffic, there is nothing they could do about it anyway.

The Najura session was said to be attended by middle-level liaison officers representing each country, and an American lieutenant colonel representing the U.N. peacekeeping force.

The Christian units, made up of the remnants of the former Lebanese army, have been controlling three semi-autonomous enclaves in south Lebanon and reports from there say that they have raised their flags over positions abandoned by the Israelis during the phased withdrawals.

The Defense Ministry officially maintains that Israel had not occupied the Christian enclaves and, therefore, those areas are not Israel's to turn over to the United Nations. Territories that Israel has maintained control over, the Israelis say, will be turned over to the peacekeeping force, as agreed.

Deputy Defense Minister Mordechai Zipori was quoted in the Jerusalem Post as saying that the U.N. forces role is to keep Palestinian terrorists out of the area, and not to bother the Christians.

The U.N. peacekeeping mission is particularly sensitive to appearing to favor any side in the dispute over control of the invaded areas, and is cognizant of the sizable bloc of sympathy the Palestinians hold in the Security Council.

The peacekeeping force has estimated that between 150 and 200 guerrillas have infiltrated across the Litani River and U.N. officials stress that completely sealing off the river would require secure fences and electronic detecting devices.