Prime Minister Menachem Begin said today that U.S. special envoy Philip C. Habib has promised Israel he will seek an "unequivocal commitment" by Friday from the trapped Palestinian guerrillas in West Beirut that they are willing to leave the besieged city.

Habib, who returned to Beirut this afternoon after a six-day tour that took him to Arab and European capitals as well as Jerusalem, apparently will seek that commitment in order to buy more time to implement his latest ideas for a peaceful resolution of the Lebanon crisis.

According to Israeli press accounts, Habib's latest plan would involve the initial evacuation from Beirut of a small number of the guerrillas in order to satisfy Israel's demand for "concrete" evidence that the Palestine Liberation Organization is willing to leave the western sector of the city peacefully.

This first step, to be accomplished while Israeli troops remain in place around the city, would be followed by the withdrawal of the Israeli Army from the outskirts of the city, the arrival in the area of a multinational force, including U.S. troops, and the exit from Beirut of the remaining PLO forces.

Begin's statement that Habib would seek by Friday to "clarify" PLO intentions was not taken here as an Israeli deadline for a break in the negotiations. Begin is scheduled to visit Zaire next week while Foreign Minister Yitzhak Shamir is to be in Washington at the same time.

It is widely assumed here that Israel will take no drastic military steps against the PLO until the two men return. Begin disclosed Habib's plan during a speech this morning to a group of Israeli academics who support his government's policies in Lebanon.

"We have a problem," Begin said. "Last night we met with Mr. Philip Habib. He did not have much good news."

Begin noted that Habib had been to Egypt and had met in London with Jordan's King Hussein, adding that those two countries were "willing to accept a portion of [the PLO guerrillas] and other countries are willing to accept another portion."

According to an account in the afternoon daily Maariv, the guerrillas would be taken to a number of other Arab countries, including Syria, Jordan and Egypt, all of which have agreed to accept them.

But, Begin said, "Until now, it is not clear even if the terrorists are truthfully willing to leave Beirut. He Habib will clarify that in the next two days, so he promised us. He said he must get an unequivocal commitment that they will leave."

Begin spoke in Hebrew but switched to English to say the words "unequivocal commitment."

It was not clear tonight what steps Israel might take if Habib fails to win from the PLO the kind of commitment the Begin government is demanding. Nor was it clear how the PLO would react to the proposals outlined in the Israeli press.

Meanwhile, Sen. Paul Tsongas (D-Mass.), who met here with Defense Minister Ariel Sharon today, quoted Sharon as saying that he considered the recent Israeli bombings of West Beirut a form of "pressure" on the PLO to force a break in the deadlocked negotiations.

The tone of Begin's remarks suggested that he and other top Israeli officials remain deeply skeptical that the PLO really intends to withdraw peacefully from West Beirut. Prior to Habib's arrival here, Israeli officials described themselves as "skeptical" and "pessimistic" that Habib's mission would be able to avert a bloody showdown with the PLO on the streets of Beirut.

But at the very least, Habib clearly won more time for his negotiations during his meetings with Begin and other Israeli officials.

"We are still stretching the time option," said one official who briefed foreign correspondents on the situation.

The official described Habib as "optimistic" and added that the American envoy "has reason to believe his assignment can be carried out successfully and that he can conclude the whole thing with positive results. We are giving him the chance to carry on and to conclude his mission."

Before he left Jerusalem, Habib met for two hours with opposition Labor Party leader Shimon Peres. Peres described Habib's attitude as "very sober" about the difficulties of reaching a diplomatic settlement, but added that "the political option stands a fair chance and all of us have to support it seriously and completely."

U.S. Sen. Tsongas, who emerged from a meeting with Begin yesterday describing "a sense of resignation about the military option," met later last night with Habib. Today Tsongas sounded slightly more optimistic.

He said he was encouraged that at least the negotiations appeared to be narrowing for the moment to the immediate question of the fate of the PLO forces in Beirut. Tsongas said that if PLO Chairman Yasser Arafat "is willing to have this settled without the [overall] Palestinian question being addressed, they might pull it off."

[At the United Nations, United Press International reported, Israel rejected a French-Egyptian proposal for simultaneous and mutual recognition between the PLO and Israel.]