Syria today rejected what it said was an Israeli demand that Syrian forces withdraw from the Beirut area. The Damascus government indicated that its troops would help defend the Lebanese capital and the Palestinians trapped there against an Israeli attack.

While it was not clear what forces Syria has left in Beirut to back up such a warning, the Syrian announcement today on Damascus radio pointed to a potential breaking point in the four-day-old cease-fire in Lebanon between Syria and Israel.

Another strain stems from Israel's noncompliance so far with an unpublicized Syrian condition for the cease-fire, according to an informed source. He said the Syrians accepted the cease-fire on terms of Israel's immediate withdrawal to within the 25-mile zone in southern Lebanon it said it wanted to clear of Palestinian guerrillas. Publicly, Syria has said it agreed to the cease-fire "on the basis of a complete Israeli withdrawal from all Lebanese territory," but no details of the pullout envisaged by Damascus have been disclosed.

According to Western diplomatic sources here, much of Syria's 85th Brigade was in the Beirut area last week. It is unclear, however, whether elements of the brigade are still positioned in the Beirut area. If it were at full strength, the 85th Brigade would count about 2,500 troops and 90 tanks, according to diplomats here.

There was no confirmation here tonight of Beirut radio reports of a tank battle between the Israelis and the Syrian troops.

According to a report reaching here from Beirut, Syrian troops were still battling alongside Palestinian guerrillas against the Israelis on the outskirts of the capital at the weekend, despite the Syrian-Israeli cease-fire.

"The Syrians will do everything they can to avoid being pushed out of Beirut," said a Western diplomat. He said, however, that Damascus considered the Bekaa Valley of far greater strategic importance and that the best Syria could probably expect from the situation now would be to maintain its presence there.

Damascus radio said the nominal commander of the Syrian forces in Lebanon, Gen. Samir Khatib, had received an Israeli demand that the Syrians leave Beirut and the surrounding area.

The radio said Khatib, a Lebanese who is theoretically in charge of the essentially Syrian Arab Deterrent Force in Lebanon, replied that it was not in Syria's power to order such a pullout. The radio said Khatib replied that the Syrians were there at the behest of Arab states and came under the authority of Lebanese President Elias Sarkis.

The broadcast said that the Syrian force would continue to "carry out its duty to defend the rule of law in Lebanon, Lebanese land and the Palestinian people in Lebanon."

The statement referred to an Arab League mandate under which a multinational force was formed to keep the peace in the faction-ridden country after Syrian troops invaded to halt the 1975-76 Lebanese civil war.