A new round of heavy fighting flared in Beirut today, breaking after less than 24 hours the fifth cease-fire of the month-old Israeli invasion. Among the targets hit in the intensive shelling were the grounds of the presidential palace and the nearby U.S. ambassadorial residence.

As the Israelis poured tank and artillery fire onto Palestinian targets from hillside positions southeast of the capital, the Palestinians fired back with artillery and Soviet-made Katyusha rockets.

Some of the Palestinian shells and rockets landed near the Baabda presidential palace, an area where the Israelis have big gun emplacements; the U.S. ambassador's residence is nearby.

Although spokesmen for the Palestine Liberation Organization denounced elements of President Reagan's offer to send U.S. Marines to assist in the evacuation of Palestinian guerrillas trapped in West Beirut, the umbrella guerrilla grouping appeared willing to continue negotiations that could involve U.S. participation in an international peace-keeping force to supervise a disengagement of Israeli and Palestinian forces.

Israeli officials, meanwhile, said that the U.S. willingness to station troops temporarily in Beirut could smooth the way for an agreement linking the departure of PLO guerrillas with a limited Israeli troop pullback, Washington Post correspondent Edward Cody reported. Details on Page A13

Today's shelling, which began in midafternoon and continued well into the night, came as Israeli troops tightened their vise around besieged West Beirut and closed off the last route through the port.

At the instigation of special U.S. envoy Philip C. Habib, the Israelis released key pieces of waterworks machinery in Christian East Beirut, which had deprived all of the western sector and a good part of the eastern sector of water.

In Washington, the State Department appealed publicly yesterday for the restoration of power and water to Beirut and the protection of the civilian population there.

Official sources said the United States had intervened with Israel on Sunday and again Monday on the same points, and by Monday night had reason to believe that Israel had agreed to restore essential services. But Arab diplomats, who had been active in generating the U.S. action, reported early Tuesday that the services had not been restored, and called for further U.S. steps.

State Department officials said power and water had been restored by Tuesday afternoon. They conceded that the overall Israeli military pressure remained very strong.

Israeli tank and artillery fire hit at least one Palestinian ammunition depot in the Burj Barajneh Palestinian refugee camp near the Beirut airport, lighting up the darkening sky with flashes of flame and causing secondary explosions. The Israelis also hit a target near the UNESCO building on the southern edge of West Beirut.

At one position on the road to Baabda, a complex of tall buildings still under construction, at least three Israeli soldiers were wounded by a Palestinian shell.

According to the radio of the rightist Christian Phalangist militia, Israel's de facto Lebanese ally here, as many as three ammunition depots were hit today. The radio later reported that a fuel storage tank was hit in a Palestinian-controlled neighborhood south of the Corniche Mazraa, which marks the southern edge of West Beirut. As night fell and a full moon rose in the east, columns of smoke wafted up from numerous points all across the southern suburbs of Beirut.

Saeb Salam, the veteran former prime minister and mediator between the Americans and Palestinians, said special envoy Habib had told him yesterday he was "nearing agreement" on a plan to solve the crisis in Beirut and avoid an all-out Israeli assault. But many loose ends appeared to remain, one indication of which was that Lebanese and PLO officials were meeting again tonight.

Bassam Abu Sharif, a ranking member of the Radical Popular Front for the Liberation of Palestine, a member of the PLO, told a news conference in West Beirut today that the U.S. offer was "ridiculous." He added, however, that the PLO would not rule out U.S. participation in a United Nations force to supervise a disengagement in the war-torn western sector of the Lebanese capital.

Although initially taken by surprise by the American plans announced today, a senior PLO official tried to make it appear that everything had been settled. The official, who declined to be identified, said that French and American military units would separate the combatants. The PLO was to maintain a political presence and its refugee camps were to be secured, he added.

The Israelis would pull back to allow the Palestinian guerrillas to pass freely without crossing Israeli lines to Tripoli in northern Lebanon, the eastern Bekaa Valley and the Syrian capital of Damascus, the official maintained.

In Cairo, Egyptian Foreign Minister Kamal Hassan Ali told the official Middle East News Agency that the PLO has agreed to leave West Beirut by sea to "avoid destroying the city and shedding more blood," and that France would help transport the PLO out of Beirut, United Press International reported.

The French Foreign Ministry in Paris said in a statement that any agreement is contingent on the "explicit agreement of the PLO and a general agreement by all parties concerned."

Such a disengagement of forces would meet PLO demands for an Israeli withdrawal.

Some PLO guerrillas and leaders would go to other countries on ships. But the official provided no indication of which countries the PLO had in mind, how many men would leave or the time frame for these operations.

Almost all the points the official mentioned have been reported since Saturday night when PLO Chairman Yasser Arafat signed for the first time a document expressing his willingness in principle to leave Lebanon with his troops.

Habib subsequently held two meetings with David Kimche, the Israeli Foreign Ministry director general. Those meetings, the second of which was attended by Defense Minister Ariel Sharon, were considered further proof that the negotiating gap was narrowing. Still, Israel had rejected previous PLO suggestions for an international force to provide guarantees because the guerrillas do not trust the Lebanese Army, which they consider too favorable to their Christian militia foes.

Such a force also had been rejected by President Elias Sarkis, Foreign Minister Fuad Boutros and right-wing Christian militia commander Bashir Gemayel. Their silence on the proposed American plan appeared to reflect confusion in their camp at President Reagan's decision.

Within an hour of the original leak by Israeli radio, the Palestinian news agency Wafa quoted an official spokesman who said nothing about the international force, but assailed the idea of any evacuation, especially aboard ships of the U.S 6th Fleet, which flies the "flag of a party to the ongoing massacre of the Palestinian and Lebanese peoples."

Other Palestinian officials at that early point scoffed at the reports, condemning them as "black propaganda intended to demoralize and demobilize our fighters."

As reported by one official, Arafat himself was taken by surprise when informed of the Israeli report and disclaimed any knowledge of the American plan. Further complicating the situation, especially if the Egyptian suggestions that the guerrillas would be taken by sea to the Syrian port of Latakia were correct, was a Damascus Radio broadcast refusing Syrian cooperation in the American plan.

Meanwhile, on the fourth day of the Israeli blockade of West Beirut, political and military groups began organizing garbage removal, flour deliveries to bakeries and creation of small clinics.

Food was said to be in adequate supply, but only 10 days' flour was on hand. Oxygen was in short supply and gasoline practically unavailable.