U.S. efforts to maintain a cease-fire and avert an Israeli assault on Palestinian guerrillas trapped in West Beirut received major setbacks today as Lebanese politicians failed to agree on a new government. In addition, Syria and Palestinian radicals rejected efforts to get them to abandon their military positions in Beirut.

With a 48-hour truce negotiated by U.S. special envoy Philip C. Habib technically in effect as of noon today, Lebanese leaders were busy negotiating the composition of the proposed National Salvation Committee and the deployment of the Lebanese Army inside the city.

There was no sign of an agreement, and a number of the parties involved in the tortuous negotiations had toughened their stand. The general feeling here was one of fading hopes for a disengagement of the many forces facing each other in and around Beirut.

The U.N. Security Council granted a two-month extension to the 7,000-man U.N. peace force in southern Lebanon and reaffirmed its earlier cease-fire resolution demanding the immediate and unconditional withdrawal of Israeli troops from the country, Special Correspondent Michael J. Berlin reported. Details on Page A21.

Usually reliable Lebanese newspapers reported that President Elias Sarkis had sent a request to Syrian President Hafez Assad to pull all Syrian troops out of West Beirut. The reports said Assad had rejected the request on the ground that only an Arab summit could decide to end the peace-keeping mandate given the Syrians as an Arab Deterrent Force. Fewer than a thousand Syrian soldiers are estimated to be in Beirut.

Far from accepting the Israeli, U.S. and now Lebanese demands for a withdrawal from Beirut, Syria has been reinforcing its positions along the highway between Beirut and Damascus and moving in more armor for a showdown.

Journalists visiting the border saw some T34 Syrian tanks moving into Lebanon, but the age and small number of tanks involved did not indicate that a major reinforcement was under way, Washington Post correspondent William Branigin reported from Damascus.

The truce arranged by Habib's efforts reduced reported fighting today to sporadic Israeli shelling of Palestinian camps south of the capital. But Israel continued to move tanks, artillery and troops into positions for a possible assault.

The Palestinians showed no sign of giving in to Israeli military pressure on their 5,000 to 6,000 guerrillas spread across the 10-square-mile area of West Beirut.

At a news conference, George Habash, leader of the Popular Front for the Liberation of Palestine, rejected the idea of his guerrillas or those of the Palestine Liberation Organization laying down their arms in the present circumstances.

Habash's Popular Front is the second strongest group in the PLO structure after Yasser Arafat's Fatah. While he generally is regarded as more radical, his views today on the disarming of the guerrillas did not seem to differ from those of relatively moderate PLO leaders like Arafat.

PLO second-in-command Salah Khalef, code-named Abu Iyad, also rejected a laying down of arms, United Press International reported. "If need be, we shall fight from our camps alone for the world to know that no one can disarm this revolution with its will and approval," he was quoted by UPI as saying.

Habash, replying defiantly to a question at the news conference, pounded the table and said, "What do you mean, lay down arms? Never. It's impossible. It's a dream. We will never stop fighting until our goals are achieved."

He did say, however, that the PLO would be willing to discuss with the Lebanese "progressive movement" a "suitable" status for the Palestinians, "after an Israeli withdrawal from Lebanon." By "progressives," he was apparently referring to the Palestinians' traditional leftist allies.

The Israelis, meanwhile, continued to move tanks, armor, artillery and troops north along the coast toward the capital and into the mountains east of it, according to eyewitness accounts in the area. This appeared to be in preparation for an assault on the Palestinian camps and military positions in and around West Beirut.

On the political front, Walid Jumblatt, the Druze leader of the leftist National Movement, still showed no sign of being ready to join the seven-man National Salvation Council even after a day of talks here with various Lebanese faction leaders.

The sole holdout, Jumblatt, yesterday was brought through Israeli lines from his home in Moukhtara in the Chouf region in a U.S. Embassy car to the presidential palace at Baabda on the hills east of Beirut.

He has single-handedly kept the salvation council from meeting, arguing that it should be enlarged to include at least four other Lebanese notables to make it more representative.

He said today he was not opposed "in principle" to the council, but now wanted 48 hours to obtain "certain guarantees" from the PLO, the Americans and other warring factions before joining it.

Otherwise, he would withdraw from politics, he said, "because I cannot bear to be responsible today or tomorrow for the death and destruction in Beirut."

Tonight, Saeb Salam, 77, a former prime minister instrumental in trying to get the council on its feet, reported that he had made "no progress" today. After repeatedly meeting for five days now with Habib, Arafat, Jumblatt and all other key political figures, Salam said in an interview at his home, "Unfortunately, I have not succeeded."

He indicated there were various reservations on many sides but seemed particularly irritated by Syria.

"The Syrians are provoking everybody now," he remarked.

Despite Israel, the United States and even now many Lebanese factions pressing for an end to Syria's special military and political role in Lebanon, Syrian President Assad apparently has decided to hold on here no matter the risk or cost to his government.

Reports from Damascus said Assad had sent a message to Sarkis yesterday rejecting his request for a Syrian troop withdrawal from the capital and questioning whether he was free to make such a request in light of the Israeli occupation.

Palestinian communiques reported that Israeli tanks and infantry tried to move farther up the Beirut-to-Damascus road but were repulsed by Palestinian guerrillas and their leftist Lebanese allies.