Palestine Liberation Organization Chairman Yasser Arafat and Syrian President Hafez Assad met formally today for the first time since September, and spokesmen for both sides indicated afterward that they had ended the strain in their relations.

Khalil Wazir, Arafat's deputy, said after the four-hour meeting in Damascus that Syria and the PLO agreed to coordinate their military resources to face "the Israeli buildup and the Israeli threats that have reached their peak" in eastern Lebanon's Bekaa Valley.

Disputes between Arafat and Assad played a major role in causing the collapse last month of talks between the PLO chairman and Jordan's King Hussein on allowing Hussein to respond positively to President Reagan's Middle East peace initiative. Pro-Syrian, hard-line factions in the PLO strongly opposed the Reagan plan.

The public statements gave no indication of whether Arafat was moving toward Assad's rejectionist position or whether Syria was becoming more moderate. A PLO shift toward the Syrian stance would indicate that the PLO was becoming more radical and thus less likely to cooperate with U.S. efforts to revive the Reagan plan.

The Arafat-Assad meeting also was seen as likely to strengthen the roles of the PLO, Syria and the Soviet Union in the diplomacy surrounding Secretary of State George P. Shultz's effort to obtain withdrawal of Israeli, Syrian and PLO forces from Lebanon.

Lebanon's foreign minister said after returning from talks with Assad that a withdrawal agreement was "not at hand." He left the impression that Syria had put heavy pressure on Lebanon not to make concessions required by Shultz to win Israel's agreement to withdraw its forces. Details on Page A18.

Shultz has said he would like to visit Syria, but U.S. officials here said that such a trip is unlikely unless he has concrete proposals for an Israeli withdrawal.

Arafat and Assad last met more than seven months ago after the evacuation of PLO fighters from Beirut. Relations were bitter then, with Arafat publicly charging that Syria had failed to aid the PLO adequately during the Israeli invasion of Lebanon and siege of Beirut.

Relations were strained further when Arafat entered negotiations with Hussein. Relations are poor between Syria, a Soviet ally, and Jordan, more aligned with the West.

Wazir, who also is known as Abu Jihad, was quoted by The Associated Press as saying, "Today's meeting ushered in a new era of Syrian-Palestinian union required to cope with the hardships facing the Palestinian revolution, Syria and the entire Arab world . . . . We consider the strain over."

A Syrian government spokesman said the talks had focused on "the situation in the area in light of the Zionist and imperialist plot to liquidate the Palestinian cause and impose hegemony on Lebanon, and on the necessity of confronting these plots and foiling them."

Local radio reports quoted Syrian officials as saying after the meeting that Assad gave strong support to the PLO's current leadership and to PLO aspirations for creation of a Palestinian state.

The Arafat-Assad meeting comes as both Syria and Israel are building up forces along their front lines in eastern Lebanon and as both are talking about the threat of a new war.

Syria has between 30,000 and 40,000 troops in eastern and northern Lebanon. In addition, an estimated 6,000 to 8,000 PLO fighters are behind Syrian lines together with a score of other irregular armed leftist groups. But an official of one of the PLO factions, the Marxist Democratic Front for the Liberation of Palestine, said in an interview in his party's magazine that "coordination is lacking" among the groups. He said that was because of the strained relations between Syria and the PLO, expressing hope that the two would soon "stand in the same trench together."

Most of the PLO fighters interviewed by the magazine said they thought war with Israel was "inevitable."

Arafat has been in Damascus since Sunday, meeting with members of the PLO's executive committee.

It appeared that Arafat, in repairing relations with Assad, did not want to break with Jordan. He sent his political adviser, Hani Hassan, to Amman yesterday with a letter for Hussein.

Arab radio stations quoted Hassan as saying that "agreement has been reached to maintain contacts and consultations between Jordan and the PLO on all future Middle East developments."