The Reagan administration continued its preparations for a U.S. military role in Beirut yesterday despite a warning from Soviet President Leonid I. Brezhnev and intensified domestic discussion about the issue.

The White House cautiously described the plan for involvement of 800 to 1,000 U.S. Marines as "a contingency," and doubts were expressed at the Pentagon about successful completion of the complicated negotiations that are to precede the U.S. operations.

Nonetheless, administration officials said plans are proceeding for participation by U.S. troops in a multinational force to safeguard withdrawal from Beirut of Palestine Liberation Organization fighters and assist a transition to Lebanese government control.

The Western White House and the State Department acknowledged that Brezhnev's letter had been received, but neither announced a U.S response. The letter was at least the third from Brezhnev to President Reagan since Israel invaded Lebanon a month ago but is the only one made public by the Soviet Union.

As interpreted by administration specialists, Brezhnev's message was neither surprising nor particularly strong.

The Soviets are always sensitive to U.S. military activity in the Middle East, which is much closer geographically to Moscow than to Washington, officials pointed out, and in this case the sensitivity may be heightened by the losses suffered by the PLO and Syria, both of which rely on Soviet weaponry and support.

Brezhnev said in the letter that in the event of U.S. military involvement, "the Soviet Union will construct its policy in accordance with this fact." This statement, described by the Soviet news agency Tass as a warning, was considered here to be just about the minimum that Moscow could say, if it chose to say anything.

Officials were intrigued by the fact that the document was made public via Tass within a few hours of its delivery Wednesday by Soviet Ambassador Anatoliy F. Dobrynin to Acting Secretary of State Walter J. Stoessel. This seemed to the officials to indicate that it was intended as much or more for effect in the rest of the world, especially the Arab world, as on the United States.

U.S. intelligence has reported more than 30 Soviet transport aircraft flights to Damascus since the Israeli invasion as well as the docking of several Soviet transport ships at a Syrian port. The contents are believed to be military supplies to replace Syrian war losses.

A Red Army division in the southwestern part of the Soviet Union reportedly was placed on alert early in the current Lebanese conflict. Some U.S. officials speculated that the Soviets might take symbolic military steps, such as sending a battalion of troops to Syria, in case the U.S. military forces go to Lebanon to assist in evacuating the PLO.

A message from Brezhnev to Reagan on the night of June 9, shortly after the Israeli Air Force destroyed the Soviet-supplied Syrian anti-aircraft missiles in Lebanon, reportedly expressed serious concern that "a most serious situation has been created which entails the possibility of wider acts of hostility."

Reagan quoted that much of the Brezhnev message in a personal missive of his own to Israeli Prime Minister Menachem Begin several hours later, asking for an immediate Israeli cease-fire.

Another message from Brezhnev to Reagan came several days later, as Israeli forces first moved up to Beirut and applied intense military pressure on the remaining PLO forces. That message was made known by then-secretary of state Alexander M. Haig Jr.

In both cases, Reagan's replies to Brezhnev went unpublished. Deputy White House Press Secretary Larry Speakes said early yesterday that no response has been dispatched, but he left the clear impression that Reagan would reply.

About the plan for U.S. military involvement, which was first made public in Israel Tuesday and confirmed later that day by Speakes and Reagan, the White House spokesman would say only, "It remains a contingency, and it is all subject to the outcome of the talks as they proceed in Beirut."

Pentagon spokesman Henry Catto, in a news briefing here, seemed much more dubious than other officials have been about prospects for the multinational operation.

"I say it's in doubt. We don't know if there's going to be a landing or not," Catto told reporters. His doubts, he added, were about whether the operation could be arranged because "there are just so many, many angles to it that have not apparently been ironed out . . . it is a hellishly complicated situation."

The Defense spokesman's doubt followed comments Wednesday by Defense Secretary Caspar W. Weinberger, who acknowledged risks to U.S. forces that might be involved and seemed anything but enthusiastic about the prospect.

At the State Department, an official commented that, "as far as we're concerned, we are marching to the president's orders" in supporting the plan. He added that the notes of disquiet from the Pentagon "may be reflecting military reservations."

According to the State Department, there is agreement on general outlines of the diplomatic settlement that would bring PLO fighters out of Beirut, under the eye of U.S. and French troops and with assurances of an Israeli pullback from the Lebanese capital. But officials said some details are being worked out in laborious and time-consuming discussions.

Moshe Arens, Israel's ambassador to the United States, told wire-service reporters here yesterday that the offer of American troops was a U.S., not an Israeli, idea. "I'm not aware of any change of heart or change of mind on this score at the present time," he said.

Secretary of State-designate George P. Shultz, who is acquainting himself with diplomatic issues and organization in preparation for his Senate confirmation hearing next week and eventual assumption of the job, was briefed for 90 minutes yesterday by Nicholas Veliotes, assistant secretary of state for Near East and South Asian Affairs.