Israeli and Syrian troops faced off across their common border yesterday in a show of strength precipitated by Syria's five-day bombardment of the Christian quarter of Beirut.

Reacting to thinly veiled threats of intervention by Israeli and the rooftop level overflight of Beirut Thursday by Israeli jet fighters. Syria deployed troops along its border near the Golan Heights and put its airport and anti-aircraft units on alert, apparently anticipating air attacks either on military posts in south Lebanon or on artillery brigades surrounding Beirut's besieged Christian sector.

The Israeli army reinforced its military units along the Lebanese border and moved units stationed in northern Israeli closer to Lebanon in a move reminiscent of many previous confrontations with its Arab neighbors.

The United States and the Soviet Union were reported to be working behind the scenes to defuse tensions and preclude the latest Lebanese crisis from erupting into a full-scale Middle East confrontation.

Israeli officials said yesterday's deployment was less a strategic maneuver than another signal to Syria that it will not tolerate a movement of Syrian peacekeeping forces farther south in Lebanon.

Israeli military officials would not say how many army units are involved or where they were moved.

Israeli government officials toned down noticeably their blunt threats of Thursday, apparently by way of waiting to see whether the Syrian army renews its shelling of East Beirut.

Moshe Arens, head of the Israeli parliament's Foreign Affairs and Defense Committee, said in Tel Aviv that "all necessary action" should be taken by Israeli to prevent a takeover of Lebanon by Syria and to put an end to the killing of Christians in Beirut.

Arens said that if Syria, which was invited into Lebanon 2 1/2 years ago by the Lebanese government to serve as a peacekeeping force during the 1975-76 civil war, succeeds in destroying the Chirstian forces, a new frontier would be created on Israeli's border.

If the Beirut situation continues to deteriorate, Arens said, it should be clear to the world - and particularly to the United States - that Israeli will "no remain indifferent."

Aren's statement, the only public warning issued yesterday by a highlevel Israel official, was low-key compared to the threats made Thursday by several top government officials. Then, Eliahu Ben-Elissar, director general of the prime minister's office, said that if the Syrians continued the bombardment, Israel would "consider steps to be taken in order not to let the Christians be annihilated."

It was then that seven Israeli Kfir jet fighters made two supersonic passes low over Beirut, shattering windows. During the Lebanese civil war, similar overflights were made over Beirut by the Israelis.

Israeli officials said that in addition to their humanitarian concerns for the Christians - who have been allied with Israel for a long time - they were concerned that a Syrian takeover in Lebanon would change the balance of forces in the Middle East and result in an increased security threat to Israel.

Israeli's close attachment to Christian Lebanese forces has been apparent for a long time, but not as much as when the Israeli army withdrew from south Lebanon on June 13 following the invasion a month before and turned key positions over to right-wing Christian militias, against the wishes of the United Nations.

That step led to suggestions that Israel intended to circumvent the United Nations and retain control over the border area by proxy, since it is known to supply the Phalangist forces with arms and money.

A possibility raised by the threat of intervention is that of an internationalization of the Syrian-Christian conflict, which ultimately could leads to a partition of Lebanon into Christian and Moslem sectors.

The secretariat of Israeli's Democratic Movement for Change Party yesterday suggested such a step when it called on the government to demand U.N. Security Council action in the Beirut conflict.

Meanwhile, Christian military leaders in Beirut said that 450 Christians have been killed in the recent attacks and 1,400 have been wounded. A spokesman for the Phalangists said that the Christians, surrounded by approximately 20,000 Syrian troops, could hold out for only a few more days.

Peres insisted he would conduct no negotiations with Sadat. "I would rather describe the meeting as a dialogue. I am not coming with my own plan," the Israeli Labor Party leader said.

Meanwhile, qualified sources said the United States has set July 18 as the opening date for talks on the Middle East in London among the Egytian and Israeli foreign ministers and Secretary of State Cyrus Vance. The talks will be the first face-to-face peace negotiations between Israel and Egypt since January.

Peres called the new Egyptian proposal "incomplete."

"It describes what Israel should do but it does not say what Egypt would do in return," he said. "There is practically nothing in it about peace and normalization."

Peres said the initiative for his meeting with Sadat came from Austrian Chancellor Bruno Kreisky and Socialist International president Willy Brandt of West Germany. The Egyptian leader is to meet with both Kreisky and Brandt during his five-day visit here.

It will be Peres' second meeting with Sadat. They met first at Salzburg, Austria, in February.