Syrian troops have lost ground in their five-day battle to keep Israel from gaining control of the Beirut-to-Damascus highway, and Western diplomatic and military analysts here said the Israelis are poised to press east toward Lebanon's Bekaa Valley.

A push toward the heavily fortified valley would be designed to force the mainly Syrian Arab Deterrent Force back into Syria, these sources said.

The sources estimate the Israeli invasion force at about 100,000. Its size, coupled with comments today by Israeli Defense Minister Ariel Sharon and Israel's demand that Syria withdraw all of the estimated 40,000 troops Western analysts say are now in Lebanon, suggest that Israel is prepared to force the Syrians out.

There was no immediate response here to Israel's announcement that it had agreed to a cease-fire in Lebanon.

An Israeli move into Syria undoubtedly would provoke a much sharper international reaction than the siege of the Palestinians in West Beirut has. Soviet sources have told Washington Post correspondent Dusko Doder in Moscow that they see Syria as a strategic ally and would move to protect Syrian President Hafez Assad's government against direct Israeli attack. This kind of Soviet commitment has not been extended, apparently, to Syrian troops in Lebanon.

Arab countries might also react more sharply. Western sources have been told that if Israel goes into Syria, Saudi Arabia would consider an immediate shut-off of the 7.5 million barrels of oil a day it is exporting.

Diplomatic sources here say the Israelis want to establish a Lebanese government acceptable to Israel and free from Syrian control. Syrian forces occupied Lebanon in 1976 during the height of the Lebanese civil war. Since then, they have influenced both Lebanese politics and the Palestine Liberation Organization in Lebanon.

Cryptic Syrian military communiques have not made it clear how Syrian ground forces are faring in their battles against the Israelis, but neutral observers said both sides have suffered substantial losses.

A Syrian military communique issued tonight said the Israelis had escalated their attacks on 16 Syrian military positions this afternoon.

Damascus has remained outwardly calm during the most recent fighting. The streets of this bustling capital were empty today as thousands of Moslems went to the city's mosques in normal observance of the Islamic holy day.

But security measures were taken around the Palestinian refugee camps in Damascus, and there were reports that the government has closed the Syrian-Lebanese border to all civilians.

A European diplomat said the Syrians were concerned about Israel's U.S.-made 175-mm guns in the southern Bekaa Valley 20 miles from here. "From where they are, those guns can easily shell western Damascus," he said.

The bulk of the Syrian forces are about 20 miles east of Beirut in the strategic Lebanese Bekaa Valley. The Syrian capital, Damascus, is only another 35 miles east of the Bekaa by the Beirut-Damascus highway, a 50-minute drive.

With another 50,000 troops believed to be near the Golan Heights, the "Syrians are stretched thin militarily," a European military analyst said. "I believe the Israelis could, with heavy losses for each kilometer, push the Syrians out of the Bekaa and back into Syria."

Another Western military source said the Israelis already have bolstered their forces in the part of the Golan Heights they annexed in December. "The Syrians would be very exposed near Golan if they began transferring troops from there to the Bekaa," he added.

Israeli Defense Minister Ariel Sharon, quoted by the BBC from inside Lebanon, said that the estimated 6,000 PLO fighters holding on in West Beirut are "on the brink of total destruction." Sharon added that only "the Syrians could have helped them, and they have lost their strategic control of Lebanon."

Some of the bloodiest fighting of the three-week-old war has taken place since Monday outside of Beirut as the Israelis have tried to capture Syrian Army positions along the Beirut-Damascus highway, a vital transportation link across Lebanon.

One high-level government official said recent Western press accounts quoting the Syrian government as saying it would withdraw its soldiers from Lebanon if the Israelis did the same were "not true."