Israeli officials said they still have not received satisfactory assurances that the Palestine Liberation Organization is willing to leave Beirut and declared that today's bombardment of the Lebanese capital demonstrated that the PLO will pay "a very high price" if withdrawal negotiations remain deadlocked.

Rejecting U.S. appeals for military restraint, an official said Israel will "hit back hard" against cease-fire violations by the PLO, a policy reflected in the air, land and sea bombardment of Beirut.

The official said the assault was "to make clear that we will not go along with a war of attrition, that there are no fixed rules to the Israeli response and that there is a very high price to be paid if they [the PLO] drag out their answer."

[In Washington, President Reagan told reporters, "I think it's absolutely imperative that the cease-fire not be violated." While not directly criticizing Israel, Reagan said, "I lost patience a long time ago. This must be resolved, and the bloodshed must stop."]

[Reagan said he planned to have "a serious discussion" when he meets Israeli Foreign Minister Yitzhak Shamir Monday at the White House. "I'll be firm," Reagan said.]

[A State Department spokesman said earlier that continued fighting "serves no useful purpose" and "makes it virtually impossible" for U.S. special envoy Philip C. Habib to secure a PLO withdrawal.]

Tonight, Israeli radio quoted sources as saying that Habib believes that negotiations over the details of a PLO withdrawal from Beirut should begin in two or three days. The radio said Habib already has worked out "withdrawal routes and schedules," but that in the opinion of Israeli sources "there is still no sign indicating this as fact."

Even as Israeli planes, tanks, artillery and warships subjected Beirut to what Israeli radio described as the "fiercest" pounding of the eight-week-old war, the government of Prime Minister Menachem Begin declared its willingness to accept another cease-fire.

Following a regular Sunday Cabinet meeting, Cabinet Secretary Dan Meridor said Begin had instructed Israeli Ambassador Moshe Arens in Washington "to inform the government of the United States that Israel is willing to accept a cease-fire in Lebanon with the explicit condition that it be absolute and mutual."

During the day, Israeli ground forces advanced for the first time in several weeks, gaining control of the Beirut airport and reaching the outskirts of a nearby Palestinian refugee camp, according to Defense Minister Ariel Sharon and Chief of Staff Rafael Eitan.

What emerged from the heavy bombardment of Beirut and the comments of Israeli officials was the clearest indication to date of the differences between the United States and Israel over how best to aid Habib's effort to find a peaceful resolution to the crisis.

The United States clearly is pressing Israel to restrain its military response to what it considers cease-fire violations by the PLO in the hope that this will help Habib in his negotiations. In a letter to Begin congratulating the prime minister on his 69th birthday yesterday, President Reagan obliquely reiterated the U.S. appeal for military restraint.

To reach the goal of "peace and security for Israel," Reagan said in the letter made public here today, "I trust you will help us ensure that Ambassador Habib is able to conduct his complex negotiations in the atmosphere he needs to achieve success."

Israel's rejection of the appeal was clear from today's bombardment of Beirut and the comments of government officials.

"I don't think an easing of the siege surrounding Beirut is the thing that Mr. Habib lacks to conclude the negotiations," one official said. "It may be the only thing that convinces them the PLO that we mean business . . . . I'm sorry, but on things like this we are not going to accept the good advice of the United States. It is our soldiers being shot at."

A senior Israeli policy-maker said the Israeli plan to "hit back hard" against cease-fire violations was made clear to Habib when he met with Begin and other top officials here last week.

Habib left here for Beirut last Wednesday after promising, according to Begin, that he would seek an "unequivocal commitment" from the PLO within two days that it was willing to leave Beirut. Officials here refused to say whether they had heard directly from Habib, but they said they had received many communications from the United States in the last few days and were aware of Habib's wishes.

As far as Israel is concerned, one official added, "There are many signs to show they do not intend to leave West Beirut despite what they say. We don't have the signs we should have had that they want to leave."

Israeli officials reacted cautiously to the Arab League's declaration last Thursday of a six-point plan for the removal of the PLO from Beirut, interpreted in some quarters as a commitment for a PLO evacuation of the city. While not rejecting the proposal outright, officials here said they would wait to hear from Habib directly before becoming less skeptical about the Palestinians' intentions.

In the meantime, as today's events in Beirut made clear, officials said the Israeli military would feel free to respond to Palestinian fire with as much force as it deemed necessary in the belief that only heavy military pressure will persuade the PLO to abandon the besieged city.

In announcing today's Israeli attack, a military spokesman said it was provoked by Palestinian fire that began at daybreak. The spokesman accused the PLO of "repeated violations" of the latest cease-fire, arranged Wednesday night, and said Israel had not responded to several of these violations, including an attack Friday night that wounded two Israeli soldiers.

"However, given the increasing number of violations," the spokesman said, "the IDF Israeli Defense Forces does not consider itself obligated to continue its strict adherence to a unilateral cease-fire."

In addition to bombing Beirut, Israeli forces, which have remained largely in the same positions around the city for several weeks, advanced today to gain complete control of the Beirut airport south of the city. The advance brought the Israeli Army to the outskirts of the Burj al Barajinah Palestinian refugee camp east of the airport, Chief of Staff Eitan said.

In an interview on the ABC-TV program "This week with David Brinkley" following today's Cabinet meeting, Defense Minister Sharon described the airport as "a kind of a no man's land" where the PLO had a tactical advantage it could exploit to fire on Israeli troops.

Sharon said he hoped the capture of the airport "will help to maintain the cease-fire," adding that today's advance was not the beginning of a gradual Israeli advance to occcupy the Palestinian refugee camps south of the city as a prelude to an all-out assault on West Beirut.

While officials here described the advance by the Israeli Army as a "local" action of no great significance, the movement of the Israeli forces to the outskirts of the Burj al Barajinah camp appeared to be part of the overall Israeli strategy of putting the PLO under the maximum military pressure short of an all-out assault.

Tonight in Tel Aviv, speaking to a group of Israeli youths who are about to enter the military, Sharon said "the hourglass is indeed running out" on Israeli patience.

"Not only shall we see that not one single terrorist will remain in Beirut, we shall also insist on the Israeli government's demand that all foreign forces must leave Lebanon, which means the Syrians as well as the terrorists," Sharon said.