Syria has installed new Soviet-supplied surface-to-air missile batteries on its border with Lebanon but has made no effort to replace launching sites knocked out by Israel last week in Lebanon's Bekaa Valley, according to diplomatic sources here.

While a five-day-old cease-fire between Syria and Israel remains fragile because of continued hostilities around the Lebanese capital of Beirut, Syria apparently has not undertaken any significant reinforcement of its Army in Lebanon since the truce, a diplomat said.

Instead, the Syrian Army has spent the past few days trying to salvage war materiel damaged in the fighting. Many damaged vehicles and other equipment have been hauled out of Bekaa across the Syrian border in recent days, witnesses said.

According to Brig. Gen. Samir Khatib, the nominal commander of the Syrian forces in Lebanon, the situation in Beirut is "degenerating" and both the Syrians and Israelis are putting up tank barricades on the Beirut-to-Damascus road.

Khatib, who arrived here yesterday from Beirut for brief consultations with Syrian authorities, said today that the two sides were about two miles apart on the Beirut-to-Damascus road and that tensions were high. He said Syrian troops were still in Beirut, but he declined to say how many.

Khatib, the Lebanese officially in charge of the essentially Syrian Arab Deterrent Force in Lebanon, indicated that Syrian troops would remain in Beirut to help defend the capital despite a reported Israeli ultimatum to withdraw.

He said he stood by Syrian statement yesterday that the troops were in Lebanon as peace-keepers at the behest of the Arab League and the Lebanese government.

Although Syrian air defenses proved practically useless against Israeli attacks last week, at least two new Soviet-supplied SAM6 batteries have been positioned near the border during the past few days, diplomats said.

According to knowledgeable sources, Syria had three SAM6 emplacements in the Bekaa proper and seven or eight more surface-to-air missile sites straddling the Syrian-Lebanese border. The Israelis struck sites well inside Syria in addition to destroying the Bekaa targets, diplomats said.

Diplomats reported indications that a limited Soviet resupply effort had begun. Soviet and Libyan transport planes have been sighted at Damascus airport, reportedly bearing military equipment.

The Lybian planes were seen unloading large wooden crates resembling those normally used to ship Mig warplanes for assembly, diplomats said.

According to a knowledgeable Western ambassador, the Soviets have agreed to replace some lost Syrian military equipment. It is not known whether Moscow is willing to supply more sophisticated weaponry than previously.