Soviet President Leonid Brezhnev said today that Moscow would support the use of U.N. forces to separate Israeli troops and Palestinian guerrillas in Beirut, but he again objected to any unilateral introduction of U.S. troops into the Lebanese crisis.

The Soviet leader, in an interview to be published in the Communist Party newspaper Pravda Wednesday, renewed his call for an international conference on the Middle East and asserted that the only "realistic" way to solve the Palestinian question "is the creation of a Palestinian state."

Brezhnev said the most pressing immediate problem was the lifting of the Israeli siege of Beirut. He continued:

"We are not against the idea that the forces defending West Beirut and the Israeli troops be disengaged as a first step. To this end one could use United Nations forces, especially as U.N. forces are already in Lebanon in accordance with a Security Council resolution.

"It stands to reason that, as before, we will categorically oppose any appearance on Lebanese territory of American forces. We have already issued a warning on this count."

Brezhnev evidently referred to his letter to Reagan on the Lebanon crisis, part of which was published by Tass on July 8 after Radio Israel disclosed that the United States was proposing to send American Marines, along with French troops, to facilitate a PLO retreat from Beirut.

Diplomatic sources in Washington report that in an unpublished portion of the letter Brezhnev informed Reagan that the Soviet Union would oppose any multinational force that did not have formal approval from the United Nations. The Soviet leader's remarks in the Pravda interview, distributed in advance by the news agency Tass, appeared to make public the sentiment he had privately expressed to Reagan.

It was unclear whether Brezhnev was seeking to rule out American participation in a U.N.-approved force.

Western diplomatic sources said that the latest Soviet pronouncement appeared deliberately vague. It contains no specific endorsement of proposals under which Palestine Liberation Organization guerrillas would leave Beirut.

The Pravda interview--a standard Soviet device for making policy pronouncements as required by a developing situation--appeared to reflect Soviet concerns that a solution to the crisis may be in the works in Washington in talks with the visiting foreign ministers of Saudi Arabia and Syria.

Brezhnev is currently vacationing at his Black Sea summer retreat and the timing of his interview indicated that the Soviets felt the need to publicly voice their concern about the Lebanese crisis and create the appearance that they are involved in the process of settlement.

Brezhnev's interview was in line with Moscow's policy of rendering full diplomatic and political support to the PLO while avoiding any direct Soviet involvement in the conflict.

The Soviet leader restated Moscow's condemnation of Israel, saying its actions in Lebanon "could not be described as anything other than genocide." He also indirectly criticized Arab divisiveness, saying "Arab unity assumes a key significance in the present situation. Those things which are hindering it should . . .be put aside at this critical hour."

But the statements did not yield any possible shifts in the Soviet position. With continued rhetorical endorsement of the PLO, the Soviets privately continue to stress that it is unrealistic to expect Moscow to support the Palestinians directly when they are still unable to muster support in other Arab countries.

All along, however, the Soviets have insisted that the Israelis would not seize West Beirut, and the longer the siege of the city continues, according to thinking here, the more unlikely such Israeli action becomes.

There is a hint in Brezhnev's interview that the Beirut stalemate may offer Moscow opportunities to reenter the Middle East peace process. But the Soviets appeared extremely cautious since they view the role of revolutionary Iran as a more destabilizing force in the region.

The outbreak of new hostilities between Iran and Iraq and the Iranian invasion of a state with which Moscow has a treaty of friendship and cooperation appears to have put fresh constraints on Soviet policy. This can be seen from the media coverage of the war, which meticulously balances conflicting reports from Baghdad and Tehran.

The Iranian challenge is seen here as more threatening for both Iraq and Syria, the latter being the pivotal ally of Moscow in the Arab world.

Brezhnev's interview, however, was devoted entirely to the Lebanese crisis, which he described as being "in the center of attention of the Soviet leadership."

His call for an international conference on the Middle East, last issued 18 months ago, was coupled with a pledge to take "practical measures" working "with all" who are ready to work for the establishment of a stable peace in the region. He said all countries of the region, including Israel and the PLO, should be joined by the two superpowers at the conference.

The renewed call appeared to reflect Soviet anxieties that Moscow may be shut out of a Middle East settlement. In this context, his interview may be either an effort to secure a piece of action for the Soviet Union in the current deliberations or possibly to encourage radical Arabs to resist the efforts by the moderates who are supported by Washington for a solution of the crisis.