Soviet President Leonid Brezhnev has called on President Reagan for "utmost" U.S. efforts "to end the Israeli aggression" in Lebanon and has warned against sending in U.S. troops, the official Tass news agency reported today.

The warning, in a personal note delivered yesterday, was vaguely worded and did not include any threats of specific Soviet countermeasures. It appeared to be a response to Palestine Liberation Organization pleas for a strong gesture of Soviet backing and was apparently calculated to place Brezhnev's personal prestige behind the diplomatic effort to bring about an Israeli withdrawal from Lebanon.

Tass said the Soviet leader told Reagan "not a single responsible statesman . . . can remain indifferent to the calls of those who are perishing at the hands of the Israeli invaders.

"In connection with statements to the effect that the United States in principle is prepared to send a contingent of American troops to Lebanon, Leonid Brezhnev warned the U.S. president that if this really took place, the Soviet Union would build its policy with due consideration of this fact," Tass said.

President Reagan said Tuesday that the United States could send troops to Beirut to help evacuate PLO guerrillas trapped there by Israel and to assist the Lebanese government in restoring order.

Brezhnev expressed the hope "at this critical moment of the events in Lebanon and around it, a sense of responsibility" would prevail "over opportunistic calculations, and that the United States would do its utmost to end the Israeli aggression." Tass added that U.S. negotiator Philip Habib should "not furnish a screen" for "the extermination of the Arab people of Palestine."

Western diplomats here said the message appeared to be in line with Moscow's policy spelled out by Foreign Minister Andrei Gromyko last Monday to an Arab League delegation that included the PLO's top political officer, Farouk Kaddoumi.

Arab diplomatic sources said that Gromyko, in response to Kaddoumi's plea, ruled out any direct Soviet involvement in the conflict but promised full political and diplomatic support.

Soviet statements suggest that Moscow now expects armed Palestinians to be evacuated from Beirut. Brezhnev's message to Reagan was interpreted by diplomats here as reflecting Soviet apprehension that an evacuation, guarded by U.S. troops, of Palestinian forces could produce political benefits for Washington.

Thus far, the Soviets had appeared to expect that irrespective of the outcome of the crisis, the United States would suffer long-term political losses in the Arab world for its support of the Israeli invasion of Lebanon.

The Soviet attitude toward the crisis has become increasingly cautious and Soviet officials speaking in private have become more assertive about Moscow's unwillingness to become entangled in the conflict.

Diplomatic analysts here say Moscow has apparently decided to cut its losses in the Middle East, justifying it on Lenin's dictum of two steps forward, one step back.

There is speculation here that since the death of Mikhail Suslov, the Kremlin's chief ideologist, the Soviets have shifted toward a more pragmatic line. The U.S. charge of a Soviet arms buildup and the Reagan administration's vow to match it, plus continued uncertainties in Afghanistan and Poland, reportedly have led to questioning of Soviet commitments around the world.

The initial Soviet reaction to the Israeli invasion of Lebanon and the threat to the PLO was cautious and slow in coming. The Soviets have regarded the PLO as a strategic asset, giving them a say in Middle East affairs. But for a capital inclined to calculate in terms of Realpolitik, the performance of the PLO must have been disappointing. Apart from a poor military showing in the face of an overwhelming show of Israeli power, the PLO has failed in a political context--it has been unable to generate support in other Arab countries.

It is possible that the Soviets now believe they have based their Middle East policy on an illusory and largely self-promoted image of PLO strength. According to this line of thinking, the current caution is based on fears of overextended commitments to Arab leaders whose standing in the Arab world itself is not clear.

Diplomatic sources say Moscow at this stage may see its interests being served by stability in the Middle East, given the role of the revolutionary Iran and its destabilizing effect on the region. Moreover, with Syria, Libya and South Yemen as its main Arab clients, Moscow lacks a platform from which it could effectively operate in the region.

There has been speculation here that the aging leadership and the poor health of key figures in the Kremlin may have contributed to a seeming paralysis of policy.

But some senior Western diplomats argue that, on the contrary, the laadership has displayed an acute sense of realism, acknowledging limited leverage in the region and the weaknesses of their former assumptions. As one source put it, "Had they been weak at the top, they could have stumbled into the crisis."