Israel has gradually reduced its military presence in Lebanon over the winter and now has fewer than 20,000 soldiers in that country, according to diplomatic and Israeli military sources.

Despite warnings by Syria and the Soviet Union that renewed fighting could erupt at any moment in eastern Lebanon, there has been no significant Israeli military buildup in Lebanon in recent weeks, the sources said. As a result, they said, the Israeli force of between 15,000 and 20,000 troops faces a Syrian force of close to 50,000 in Lebanon and along the Syrian-Lebanese border, including about 40,000 combat troops in the Bekaa Valley.

The Israeli daily Yediot Aharonoth said this week that Israeli troop strength in Lebanon had shrunk to 15,000. This figure was confirmed as roughly correct although sources indicated there may be slightly more than 15,000.

For several months the generally published estimate of the number of Israeli troops in Lebanon had been 30,000. During the height of the war, Israel had more than 100,000 soldiers there.

Military sources gave three reasons why Israel is content to have about a 3-to-1 manpower disadvantage along the line separating it from the Syrians. They said the Israeli Army holds the more defensible terrain, partly offsetting the disadvantage in numbers.

In addition, despite the installation of Soviet-made long-range SA5 surface-to-air missiles in Syria, the sources said Israel still enjoys clear air superiority, which was a major factor in last summer's fighting in eastern Lebanon. Also, the sources said, Israel is confident it will not be caught off guard by a sudden Syrian move and will have time to rush more troops into Lebanon.

The lower estimate of the Israeli military presence in Lebanon appeared to lend credence to recent statements by civilian officials here who have played down the prospect of renewed fighting with Syria.

Although officials warn that the situation in Lebanon remains dangerous, they have generally dismissed Syrian and Soviet predictions of an imminent outbreak of hostilities as political and psychological ploys designed to subvert the new Israeli-Lebanese troop withdrawal agreement and not a prelude to renewed large-scale fighting.

This impression was reinforced by a recent tour of the Bekaa Valley by Washington Post correspondent Herbert H. Denton. He reported that Syrian forces there appeared to be keeping a close check on the Palestinian guerrillas who operate behind their lines. Denton said the Syrian soldiers did not appear to be in a high state of readiness or to expect hostilities to break out.

However, the relative calm in the area was broken last night by small arms fire from the direction of the Syrian lines that killed an Israeli soldier, according to an Israeli announcement today. The soldier was the 483d Israeli killed in Lebanon since the invasion began last June 6. In the 1967 war 657 Israeli forces were killed and 2,680 were killed in the 1973 war.

According to Israel, in addition to nearly 50,000 Syrian troops arrayed against them, there are 6,000 Palestinian guerrillas in the Bekaa Valley and 2,000 in northern Lebanon.

The Syrians are also being supported by 4,000 to 5,000 Soviet military advisers and technicians, including "a few hundred" who are in Lebanon, Israeli sources say. The Soviets, however, appear to be playing a strictly advisory role as well as showing political support for Syria.

Meanwhile, the diplomatic movement toward a signing of the Israeli-Lebanese troop withdrawal agreement, probably next week, continued today. U.S. special envoy Philip C. Habib met with Israeli Foreign Ministry officials and delivered to them clarifications from the Lebanese of some of the points in the accord.

Last week when it approved the agreement "in principle," the Israeli Cabinet said it would seek the clarifications on both security and political issues. Foreign Ministry officials said the Lebanese responses were satisfactory and that there was no bar to signing.

The three negotiating teams--Israeli, Lebanese and American--are to meet Friday in the Israeli coastal city of Netanya to exchange and review the final drafts of the accord and fix the date for the signing.

The signing ceremony, however, will not bring an Israeli withdrawal from Lebanon until Syria and the Palestine Liberation Organization also agree to withdraw their forces. Israel has made this a condition for implementation of the agreement with Lebanon, leading to fears that a Syrian refusal to withdraw could result in renewed fighting between the two armies in eastern Lebanon.

Lebanese Foreign Minister Elie Salem went to Damascus Thursday for the second time to brief Syrian leaders on the withdrawal agreement and to attempt to get their approval for it as a basis for Syria to pull out its army from Lebanon, Reuter reported from Damascus. He met for two hours with his Syrian counterpart Abdul Halim Khaddam, but officials made no comment on the talks.

The Associated Press reported that Col. Mohammed Khouly, personal adviser to Assad, met with former Lebanese president Suleiman Franjieh and former Lebanese prime minister Rashid Karame amid reports that Beirut might be willing to grant Syria patrol rights similar to those sought by Israel.

Military sources said Israel began reducing its troop strength in Lebanon last fall after the PLO evacuation from Beirut and after the line separating the Israelis from the Syrians had "stabilized." They said the reduction continued steadily for three or four months until it reached the current level of less than 20,000.

Military and diplomatic sources cited domestic economic concerns as a major factor in the steady troop reduction. The Israeli Army relies heavily on reservists who are mobilized in times of emergency.

The higher the level of mobilization, the greater the dislocations in the economy as more reservists are called from civilian jobs to the Army. It was therefore in the government's interest to keep as few soldiers in Lebanon as it thought necessary.

The thinning out of the military presence in Lebanon also appears to have had a political advantage in helping to prevent more public impatience with the Lebanon stalemate.