At a time when Soviet-American relations are at low ebb, the growing US military involvement in Lebanon risks heightening tensions with Moscow even more.
Experts in and outside the administration agree that the Soviet Union does not want a confrontation with the United States in the Middle East. But they voice concern that, given the bad state of relations, US policy may push the Soviets toward taking steps to signal President Reagan he can go only so far.
At a press conference in Moscow on Monday, Soviet officials warned the US against further military involvement in the Middle East.
''There is definitely a danger now of more and more episodes in the next few days, with planes going over and retaliating,'' says Marshall Shulman, director of the Harriman Institute at Columbia University. ''The calculation of Israel and the US is that that kind of peripheral presence will force the Syrians to withdraw. But that is not a sensible assumption. The risk is of something going out of control - it's a difficult operation to calibrate. It would be better if we had direct communication with the Soviets.''
The danger point, in the opinion of Dr. Shulman and others, would be if the United States and Israel decided to push Syria out of Lebanon militarily.
Administration officials say that although the Soviet Union has rearmed Syria and reestablished a strong presence there, the Soviets in general have advised the Syrians to be cautious. They never approved of Syria getting into Lebanon, particularly since this has given the US an opportunity to establish a military presence there. And while they are willing to help the Syrians defend themselves against Israel, the Soviets are not willing to give them enough leeway to bring on a war with the United States.
''They are determined to maintain a foothold in the Middle East,'' says one US official, ''but they don't like what Syria is doing, and there is no open commitment to supply it.''
The risk lies in the fact that the Syrians act independently of Soviet advice , experts say. If the US and Syria get into a confrontation that endangers the situation, the Soviet Union might feel compelled to react.
''We're using force to run over a Soviet ally and protege,'' says Mark Garrison, director of the Center for Foreign Policy Development at Brown University and a former US diplomat. ''The Soviets will feel forced to respond and to make us hurt in some way. I'm especially concerned about a successful or partially successful attack on US ships off Lebanon.''
In adopting a policy of increased military cooperation with Israel, say experts, the Reagan administration seems to be acting on the theory that the only way to get public support at home for dealing with Lebanon is to turn the situation there into a Soviet-American confrontation.
Political calculations may now determine future US strategy. In the opinion of some analysts, the White House will be watching to see how the American people react to events in Lebanon and the continued administration emphasis on ''getting tough with the Russians.''
The US invasion of Grenada has won public approval, but it is not yet clear how Americans regard a military escalation in Lebanon. If public sentiment is adverse, say these analysts, the President may have to revitalize Soviet-US contacts and do something to take the peace issue away from the Democrats.
Shulman, who served as adviser on Soviet affairs in the Carter administration , says that in the face of a tough-line US policy - including the deployment of new theater nuclear missiles in Europe - the Soviets are turning more to military responses. He suggests that the forthcoming session of the Supreme Soviet will approve higher defense allocations in order to build new systems and says tensions with the United States will rise.
''We're in a for a long hard time - not war, but tensions - and the risk of war is somewhat higher because of less stable (nuclear) systems.''
There are indications that some administration officials believe a more assertive US military role in Lebanon is opportune now because of the reported illness of Soviet leader Yuri Andropov and of President Assad. The assumption is that neither would do anything drastic at the moment. But this view is challenged by experts as dangerous thinking, since it cannot be assumed that Mr. Andropov is not making decisions. Moreover, if he is not, experts say, it cannot be ruled out that some Soviet group might for internal reasons try to ''teach the Americans a lesson.''
Some diplomatic and academic analysts say the United States needs to make some sort of gesture toward the Soviet Union in order to increase its restraint on Syria. Others note, however, that even at the height of US-Soviet detente, in 1973, Moscow did not shy from raising tensions in the Middle East. The Syrians, furthermore, now have enough military equipment to give them a greater measure of independence of Moscow.
If better US-Soviet relations would not necessarily deter adverse Soviet action in the Middle East, however, it is felt that a more normal dialogue could help prevent an escalation of conflict to a perilous point.
(From Moscow, correspondent Gary Thatcher reports the Soviet Union has warned the US that further actions against Syria ''constitute a serious threat to peace in the Middle East, and not only in that region.''
(The warning was carried in a statement by Tass, the official Soviet news agency. The statement charged that the United States and Israel are trying to subject the Middle East ''to their military and political control.'' The statement reiterated Soviet support of Syria, and underscored comments made at a press conference by Marshall Nikolai Ogarkov, the Soviet armed forces chief of staff. He pledged to give continuing ''support and aid to those who are countering aggression in that region.''
(''The present US administration has irrevocably doffed the mask of an 'honest broker' in Middle Eastern affairs,'' Tass continued, ''and has openly opted for the course of military-power pressure. . . .''