U.S. officials yesterday gave credit for the Lebanese cease-fire to Syria's realization that it cannot keep agitating the civil war there without increasing its military involvement to the point where it would risk direct clashes with the United States and possibly provoke a new war with Israel.

Specifically, the officials said, Syria's hopes of winning control in Lebanon through its support of dissident forces fighting President Amin Gemayel's government had been thwarted by President Reagan's determination not to remove U.S. Marines from Beirut and by the unexpectedly strong showing of the Lebanese army during bitter fighting in the Beirut area. That, the officials continued, has created a military stalemate that Syrian President Hafez Assad could not break unless he increased his aid to the Druze militia and other dissident groups by throwing Syrian forces directly into the fighting.

But that would mean confrontation with the American, British, French and Italian units forming the multinational force in Beirut and, even more threatening to Assad, force Israel to consider whether it has to go to war again to prevent Syria from gaining control over Lebanon.

For these reasons, the officials said, the Syrians apparently have decided to put aside the military option, at least for the moment, and try to win greater Syrian influence in Lebanese affairs through a process of political negotiation between Gemayel and the Syrian-backed factions in the Lebanese civil war.

The officials, echoing the caution expressed publicly yesterday by President Reagan and his senior advisers, acknowledged that the cease-fire is a very fragile achievement that could unravel into renewed fighting, especially if there is not quick movement toward the negotiations on "national reconciliation."

As Reagan said yesterday in New York, the cease-fire accord is only "a first step" and there still is "a long way to go" in solving the bitter religious and political feuds that have plagued Lebanon with warfare for a decade and that threaten to dismember the country into antagonistic, foreign-dominated enclaves.

The officials, who declined to be identified, noted that, as of late yesterday, Beirut was still the scene of fighting.

They expressed skepticism about the cease-fire going into effect on schedule at 6 a.m. today in Beirut (midnight EDT). Even Reagan, in announcing the accord, pointedly told reporters, "You see my fingers are crossed."

The president's caution underscored how much the intractability of Middle East tensions has caused him to lower his expectations since Sept. 1, 1982, when he went on national television to announce an ambitious plan for resolving the Arab-Israeli conflict.

In the ensuing 13 months, the Reagan initiative has not been realized and his bold blueprint for Mideast peace has shrunk to what has looked like a desperate effort to keep a military toehold in Beirut on behalf of a Lebanese government whose authority barely extends beyond the city limits.

Still, Reagan administration officials insisted yesterday that, with luck and skill, the cease-fire agreement could mark a turning point in U.S. efforts to put Lebanon back on the road to stability and allow a renewed attempt to tackle the larger problems of the region.

They said a cease-fire covering both the Beirut area and the nearby Chouf region, where Druze and Christian militias have been battling each other, is the necessary first step in implementing the diplomatic strategy that has been pursued for the past month by Reagan's special Mideast envoy, Robert C. McFarlane, with the aid of Saudi Arabia's ambassador-designate to the United States, Prince Bandar bin Sultan.

That strategy centers on inducing Syria to stop obstructionist activities inside Lebanon and cooperate in patching together an accord that will satisfy the political aspirations of the various contesting factions, allow Gemayel to extend his government's authority throughout the country and permit the withdrawal of Israeli, Syrian and Palestine Liberation Organization forces occupying large portions of Lebanon.

The administration had hoped that it was on the way to achieving that objective in May when Secretary of State George P. Shultz helped to work out an agreement for Israeli withdrawal.

But that effort was derailed when Syria, which earlier had led the United States to expect its cooperation, rejected the accord and began agitating Druze and certain Moslem factions to oppose Gemayel's Christian-dominated government.

Earlier this month when Israel withdrew its forces from the Chouf to new positions in southern Lebanon, a vicious cycle of new fighting broke out in the Chouf and in Beirut, with the Syrians arming and encouraging the Druze and other groups like the Shiite Moslems to attack the Lebanese army.

As one U.S. official put it, "Assad believed the Gemayel government would fold, and he could just walk down the Damascus-Beirut highway behind his surrogates to take control." According to U.S. officials, a key element in the Syrian strategy was to instigate attacks on the Marines in the hope that domestic pressure would force Reagan to pull them out and rob Gemayel of U.S. support.

But, despite misgivings in Congress over Marine casualties, the officials said it has been made clear to Syria that the Marines will not be withdrawn and that the United States is prepared to use the massive firepower it has arrayed off the Lebanese coast in carefully calibrated responses to attacks against the Marines.

In addition, the officials stressed, the Lebanese army, benefiting from months of intensive training by U.S. advisers, proved able in the past month's fighting to withstand fierce Druze assaults, aided by Syrian arms and Syrian-influenced Palestinian fighters, and to keep control of the perimeters around Beirut.

Using these factors as ammunition, the officials said, McFarlane and Bandar emphasized to Syria that to stay with its military approach eventually might mean fighting the multinational force and possibly persuading Israel to send its forces north again for a confrontation that Syria would lose.

At the same time, McFarlane argued that Syria had an alternative in the "national reconciliation dialogue" offered by Gemayel to his foes.

The object would be to restructure the Lebanese political system to give more power to the disaffected Moslem and Druze factions.

With this as a vehicle, Syria, through its ties to these groups and its proposed role as a mediator in the talks, could seek to regain its former political and economic influence in Lebanon.

"It was a chipping away process aimed at convincing the Syrians they stand to gain more and safeguard what they see as their interests in Lebanon by talking instead of shooting," one official said last night. "We're still not sure if it will go that way, but the cease-fire agreement is at least an encouraging sign that they're at least willing to give it a try."