Israeli and Syrian military forces are unlikely to remain in Lebanon very long, a senior Western diplomat said today, disputing the generally held belief that both sides have dug in for a long stay.

The diplomat expressed this view in a background discussion with Western reporters even as Syrian troops reinforced their positions in Lebanon's Bekaa Valley and Israeli artillery shelled Palestinian positions in Beirut.

"I think the momentum toward an eventual withdrawal" of the Israeli and Syrian armies "is too well along" for both countries not to pull their forces out of Lebanon in the near future, he said, adding: "I don't see a partition of Lebanon by occupying Israeli and Syrian forces."

He declined to discuss American efforts to obtain the withdrawal from Lebanon of Israeli and Syrian troops.

Other Western and Asian diplomats in this capital, however, said they expect both the Israeli forces and the reinforced Syrian units to remain in Lebanon for some time. Syrian Information Minister Ahmed Iskander Ahmed recently declared that although Syria has agreed to observe the cease-fire proposed by Israel, Syria will not be pressured into withdrawing "by an Israeli decision or a foreign decision."

Israel invaded Lebanon June 6 in a well- coordinated attack on strongholds of the Palestine Liberation Organization. As Israel's forces moved north through Lebanon, they clashed on the ground and in the air with Syrian units that have been in Lebanon since Syria intervened in the country's civil war in 1976.

Both sides suffered substantial losses, diplomats here believe, but the Israelis destroyed 19 Syrian surface-to-air missile sites in the strategic Bekaa Valley, plus at least 80 Soviet-supplied Mig fighter planes in battles with Israel's American-made F15 and F16 jets.

Since the June 11 cease-fire, Syria has moved new SA6 missile batteries to its border with Lebanon and increased its troop strength--principally in the Bekaa Valley area--from 25,000 to around 40,000 troops, according to one observer. The Syrians also have dug in on the Lebanese side of the bulge that protrudes into Syria just 10 miles west of Damascus. Israeli forces are dug in in the same area facing the reinforced Syrian positions, diplomatic sources reported.

"I don't see the Syrians leaving the Bekaa Valley," said one Asian diplomat. He added that U.S. Secretary of State Alexander M. Haig Jr. "is looking for a complete Syrian withdrawal, but the Syrians see the Bekaa as their first line of defense before you reach Damascus."

"Despite their demand for a complete Israeli withdrawal," he continued, "the Syrians would prefer an Israeli presence in southern Lebanon and themselves in Bekaa, rather than leave Bekaa."

The Israelis have said that they will withdraw from southern Lebanon if the Syrians pull out of Bekaa, a proposal that the Syrians have rejected.

"I don't think the people in the Middle East are so shocked as we in Europe are about a continuing Syrian role in Lebanon," a Western diplomat observed. "The Syrians and the Lebanese do not feel that they are so separate, and they share a feeling of belonging to a greater Syria."

It was learned yesterday that there are approximately 1,000 Syrians and 1,600 Syrian-commanded Palestinians inside Beirut, alongside the PLO forces. Syrian Information Minister Ahmed said recently that Syrian troops in Beirut have orders "to resist if Israel breaches the cease-fire."