U.S. negotiators conferred with government leaders in Lebanon and Syria today in an apparent effort to overcome Syria's refusal to receive the thousands of Palestinian guerrillas encircled here by Israeli forces.

The French government, removing a major block to the negotiations, announced in Paris that it had agreed in principle to send peace-keeping troops to Beirut to serve as a buffer between the opposing forces. The French troops would be sent only with the approval of all the parties in the Lebanese conflict and of the United Nations, according to a message to the Lebanese government delivered by the French ambassador here.

The United States said Tuesday that it was willing to send forces to help expedite a Palestinian evacuation from Beirut, and there were reports at the time that France would also send troops. The government of President Francois Mitterrand has been a principal conduit to the Palestine Liberation Organization in the talks, and France's apparent hesitation last week had been viewed as one of a number of impediments to resolving the conflict.

The talks went ahead during a relative lull in combat today following one of the heaviest rounds of fighting since the Israeli invasion on June 6. Nine hours of heavy bombardment by Israeli tanks, artillery and gunboats lasted into the early morning and added more ruin and rubble to the Palestinian camps and neighborhoods on the southern outskirts of West Beirut.

The shelling also wrecked the luxury Summerland resort hotel on Ramlet el Baidah beach south of Beirut and the six-story Algerian Embassy. The shelling that hit those buildings and others in the area presumably was aimed at Syrian and Palestinian positions on the coast, but none was known to have been hit.

Palestinian artillery and rocket fire aimed toward Israeli positions at Baabda in the hills about 5 miles southeast of Beirut also missed the mark, with one shell landing in the courtyard of the nearby presidential palace and injuring a guard. A few shells landed near the residence of U.S. Ambassador Robert Dillon, but no major damage or casualties were reported.

A key figure in the negotiatons, former Lebanese prime minister Saeb Salam, accused Israeli Defense Minister Ariel Sharon of ordering the bombardment to sabotage the talks and of "terrorizing the whole of Beirut."

Israel said today that nine of its soldiers had been wounded during the past 24 hours, while there was no reliable estimate of the number of Lebanese and Palestinian casualties for the period. Officials at West Beirut's three main hospitals reported a total of 515 persons killed and 2,200 wounded in the capital since the Israeli invasion, about 90 percent of them civilians. The figures did not include persons who died and were buried without being taken to a hospital.

In Baabda, which has been occupied by the Israelis, talks continued today among U.S. special envoy Philip C. Habib, Lebanese President Elias Sarkis, Prime Minister Shafiq Wazzan and Foreign Minister Fuad Boutros. Israel's state-run television reported that Habib had set Aug. 1 as an informal target date for the withdrawal of Palestinian forces from Lebanon.

U.S. Deputy Assistant Secretary of State Morris Draper conferred with Syrian leaders in Damascus, where he reportedly met Foreign Minister Abdul Halim Khaddam.

In a statement last night carried by the official Syrian news agency, the Damascus government rejected the idea of receiving the 5,000 to 6,000 Palestinian guerrillas under a U.S. plan to evacuate them from Beirut to avert a threatened Israeli assault.

State Department officials in Washington, however, said that the Syrians, in their private talks with Draper, had not completely ruled out accepting the guerrillas in the first phase of the PLO's withdrawal from Beirut, Washington Post correspondent Don Oberdorfer reported. "They have not shut the door," an official said.

Diplomatic sources reported that Syrian President Hafez Assad is believed to have told leaders of Saudi Arabia during his recent visit there that he would cooperate in facilitating the departure of the PLO. This is believed to include taking them in at least temporarily.

Syria's public refusal to take in the PLO was greeted with some satisfaction in Palestinian circles in Beirut, where some elements of the PLO appear to strongly oppose any withdrawal. The rejection was seen as furthering efforts to play for time in the hope that a pullout could be avoided.

"I think it's a good thing," one PLO source said of the Syrian statement. "I don't think anybody wants to go to Syria to start with."

A well-informed foreign source said that "there is not very much progress" in Habib's negotiations. "In fact there is a retrogression because of the Syrian refusal." If another destination cannot be found or Damascus cannot be persuaded to reverse its decision, "we will be back at square one," he said.

Other sources close to the negotiations have said that a few other countries--notably Algeria, Iraq and Egypt--might be willing to receive the Palestinians but that the PLO itself still has not made the basic political decisions either to leave Beirut or picked a destination if it does. This appeared to contradict other reports in recent days that the PLO had agreed in principle to leave.

Some optimism regarding the talks was encouraged here, however, by France's willingness to send troops to Beirut as a peace-keeping force.

The French decision, conveyed to Sarkis today by Ambassador Paul-Marc Henry, followed a formal request yesterday by the Lebanese government for the dispatch of French troops. An informed source said that he expected agreement on the French condition for acceptance of any evacuation plan by all parties involved, including the PLO and the Israelis. The message to Sarkis also proposed that the force's composition and role should be defined precisely by a Lebanese-Palestinian commission.

In Santa Barbara, Calif., where President Reagan is vacationing, White House Deputy Press Secretary Larry Speakes said that the United States "would welcome" the use of French troops as a peace-keeping force in Lebanon but that the U.S. government had not yet received official confirmation from Paris of France's willingness to send forces.

State Department officials said that the French demand for U.N. approval of the force was not expected to create difficulties. One official said that France had suggested that it would seek "the least troublesome form" of U.N. backing.

Before the French announcement was made in Paris, the Soviet Union warned France "of the consequences to which submissiveness to Washington's dangerous plans" could lead. The warning was issued without further detail in excerpts carried by the Soviet news agency Tass of a commentary to appear Sunday in the Communist Party newspaper Pravda.

Soviet President Leonid I. Brezhnev, in a letter to Reagan made public Thursday, warned Washington to keep U.S. troops out of Lebanon.

It was understood that France would prefer to use its contingent in the U.N. force in Lebanon, the peace-keeping force formed to supervise Israeli troops' withdrawal from southern Lebanon after their invasion in 1978.

A more serious issue is when the force should arrive in Beirut--before or after the proposed Palestinian withdrawal. The question is reported to be holding up the current negotiations. Habib insists that the international force be deployed afterward and the Palestinians demanding that it must come in beforehand.

[The U.S. aircraft carrier Independence, meanwhile, steamed toward the eastern Mediterranean to join other 6th Fleet ships for possible duty if U.S. Marines are ordered to Lebanon, United Press International reported.]

[The Independence, carrying about 5,000 men, left Naples Friday and was joined later in the day by the guided missile cruiser Biddle.]