The new leader of Lebanon's Christian militias announced today that his forces will leave Jezzin, the last Christian enclave in southern Lebanon, and, at the request of Syria, he ordered the closing of a Lebanese Christian liaison office in Israel.

The moves represented a sharp reversal in the policy of the Christian militia and its leadership, in the view of observers, and a clear recognition by the Christians that Syria is replacing Israel as the dominant military power in Lebanon. There was no comment from Israel today.

Elie Hobeika, head of the Lebanese Forces and leader of a group of dissident Christian commanders, said he would welcome deployment of Lebanese Army soldiers in Jezzin. More than 60,000 Christian refugees have gathered there since late April, when Druze militiamen and their Moslem allies drove them from their homes in a push through the foothills of the Chouf Mountains to a coastal outlet at Jiye.

The withdrawal of Lebanese Forces militiamen and the Israeli-sponsored South Lebanon Army from Jezzin would permit deployment there of regular Lebanese Army troops, a move that Syria has demanded before it will intervene militarily to help end the current round of sectarian fighting that has killed at least 113 persons since April 28, the Beirut newspaper An Nahar reported today.

But Hobeika gave no timetable for the withdrawal, and there was no indication when the South Lebanon Army would pull out its troops.

Shelling between the Lebanese Forces, fighting alongside the South Lebanon Army, and Shiite Moslem militias around Shiite villages near Jezzin had caused an exodus of residents and prompted Nabih Berri, leader of the Shiite Moslem Amal militia, to threaten to bombard Christian villages.

Christian families also had fled to Jezzin from Sidon when the Lebanese Forces and the South Lebanon Army opened a new front against the Lebanese Army, Palestinian guerrillas and their Moslem allies in Sidon in March.

Christian refugees have blamed a two-month-old rebellion in the Lebanese Forces for what they fear is the permanent loss of their homes to Moslem and Druze forces.

Hobeika replaced Samir Geagea as head of the Lebanese Forces on May 9, a month after Geagea, accusing the Lebanese government of being too closely allied to Syria, revolted against President Amin Gemayel. The appointment of Hobeika, who had been the militia's intelligence chief, reportedly was made at Syria's insistence.

"In order to preserve the security of Jezzin and prevent a recurrence of what happened in the Sidon area, we welcome a quick deployment of the Lebanese Army," Hobeika said. "Orders have been given to our forces in Jezzin and the border strip, mostly logistics units, to return to their barracks in Beirut."

Hobeika's move also appeared to be a concession to Moslem demands and a show of good will. Christians remaining in southern Lebanon, however, were said to feel unprotected and betrayed by the Lebanese Forces, who had gone on the offensive in the area with the announced purpose of protecting them from the Moslems.

Jezzin is besieged by Moslem forces from the north and the east. Israeli officials have made it clear that they would not come to the rescue of Christians in southern Lebanon. Clashes increased today on the Jezzin-Kfar Falous axis.

Hobeika said instructions had been given for closing down the Lebanese Forces liaison office in Israel, a move that effectively ends 10 years of close collaboration between the dominant Christian militia and Israel.

The office had been opened a year ago, with heavy publicity from Israel, but had been used largely as a propaganda arm of the Lebanese Christians. Israel used less visible channels to communicate with the Christians, and apparently never considered the office a critical link, Washington Post correspondent Edward Walsh reported from Jerusalem.

Hobeika, through most of his military and political career, had been closely allied to Israel. But upon replacing Geagea, he announced a break, saying "crucial circumstances" during the past decade had "forced some of us to resort to certain regional powers hostile to our Arab environment." He said the reason had been "merely for self-defense" of the Christian community and that he now saw "the necessity of returning to our Arab environment."

An Israeli government commission accused Hobeika of leading the massacre in September 1982 of hundreds of Palestinians at the Sabra and Shatila refugee camps in Beirut, then under Israeli control.

Asked to explain Hobeika's shift toward Syria, Christian sources said he "will be pro-Israeli if it is good for the Christian community, and he will be pro-Syrian if it is more beneficial for the Christians."

The same sources, who refused to be identified, said Hobeika had established secret and still unofficial channels with Syria. Despite widespread Moslem objection to him, Syria has not criticized the emerging Lebanese Forces strongman.

Gemayel is to meet Syrian President Hafez Assad next week, and high hopes are pinned on those talks.