In an attempt to avert a clash between Christian militias in southern Lebanon and a 500-man Lebanese Army battalion scheduled to move south Tuesday, U.N. Secretary General Kurt Waldhem has asked Israel to help restrain the militias, government sources said today.

Tensions were reported to be rising in southern Lebanon between Christian militias, led by Maj. Saad Haddad, and the U.N. force because of the planned redeployment of the Lebanese Army unit, which Haddad says was selected for its pro-Syrian and pro-Palestine Liberation Organization leanings.

Haddad has threatened military action against the battalion and against U.N. forces if the troops move southward from the coastal city of Sidon. But the Lebanese government, which has sought to end Haddad's control in the south, said it will not be intimidated into canceling the redeployment.

The pending confrontation, coming pracitcally on the eve of a U.N. Security Council debate on the U.N. role in Lebanon, has placed the Israeli government in an awkward position because of its long-standing covert support of Haddad's militias.

Since withdrawing its troops from Lebanon last June following its invasion. Israel has been quietly supporting the Christian militias with arms and money in hopes of maintaining a cordon sanitaire along its northern border.

Israeli officials have said little about the crisis developing in southern Lebanon, to avoid the appearance of maintaining a surrogate army on foreign soil. But sorces said today that Israel will not support any military offensive by Haddad's militias as long as the Lebanese Army does not enter the Christian enclaves along the border.

The militias control a five-mile-deep strip running rom the Mediterranean to the Golan foothills. Reports from Lebanon said that schools in the area were closed today, and that Haddad's reservists were being called up in anticipation of a confrontation.

Moreover, U.N. officials said Haddad had warned that any U.N. helicopters flying over Christian-held territory will be shot down; that militias fired eight artillery shells at Norwegian, Dutch and Nepalese units of the U.N. force, and that the militias had menacingly positioned weapons on hills over-looking U.N. headquarters in Nakura.

"There are very high tensions. The militias have closed all the roads,leading to the Christian enclaves, and they have threatened to use force not only against the Lebanese Army, but against U.N. forces as well," a U.N. spokesman said today.

The commander of U.N. operations in the Middle East, Gen. Ensio Siilasvuo, met yesterday with Israeli Defense Minister Ezer Weizman to discuss the crisis, and Weizman reportedly reiterated Israel's position that pro-PLO troops cannot be deployed along Israel's northern frontier

After meeting with U.S Ambassado Samuel Lewis to discuss the situation, Prime Minister Meachem Begin summoned Foreign Minster Moshe Dayan and Weizman for talks on Israel's position during the confrontation. Details of their discussions were not disclosed.

In Washington, State Department spokesman Tom Reston, commenting on the plan to move the battalion into the south, said: "We strongly support this objective as part of the effort to restroe Lebanese government authority in the south as called for by the United Nations."

[Asked whether the United States was trying to keep Israel from opposing the move, Reston had no coment.]

Government sources said that Waldheim and several members of the U.N. Security Council had asked Israel to pressure Haddad into using restraint.

An Israel government source said, however, that if the Lebanese battalion actually has Syrian and PLO supporters, then Irael will consider the move cause for corncern.

However, the U.N. command here said the 500 Lebanese soldiers will be spread thinly through five U.N. battalions, and will be under the operationalcontrol of U.N. soldiers.

The U.N. units are situated south of the Litani River, but north of Christian enclaves controlled by Haddad. Haddad has said that the introduction of Lebanese Army units into his area will almost certainly lead to infiltration by Palestinian terrorists and will jeopardize Israel's border.

Hugo Rocha, a U.N. spokeman, said the organization is powerless to prevent the Lebanese government from moving Army units south.

"It's a sovereign decision on their part. When they reach the U.N. area of operations, of course they can count on being attached to our units," Rocha said.

Rocha said "consultations" among U.N. officials, Israel and the militias were continuing.