Syrian President Hafez Assad and a fundamentalist Islamic faction besieged by Syrian allies in northern Lebanon agreed last night to an immediate but uncertain cease-fire as the Soviet Union reportedly told its embassy here to begin evacuating dependents and some diplomats today.

There was no further information about the fate of three Soviet officials kidnaped on Monday. They were among four seized by the fundamentalist Sunni Moslem faction, which has demanded an end to the attacks against it by Syrian-allied leftist Moslem militias. One of the four was found shot to death Wednesday.

The authoritative newspaper An Nahar reported this morning that the Islamic Jihad terrorist group said it had executed kidnaped U.S. Embassy official William Buckley to avenge Israel's air raid Tuesday on Palestinian headquarters in Tunisia.

[Spokesmen for the White House and State Department said in Washington they had no independent confirmation of the report.]

Buckley, 57, the chief political officer at the U.S. Embassy here, was abducted from his car in west Beirut on March 18, 1984. He has been held the longest of the six American hostages still in Lebanon.

Islamic Jihad, a fundamentalist Shiite Moslem group with ties to Iran, recently warned through released hostage Benjamin Weir that it would kill the American hostages if Washington did not pressure Kuwait to release 17 Shiites jailed for a series of bombings in that country. The Shiite fundamentalist group is a rival of the Sunni group that claimed responsibility for kidnaping the four Soviet citizens.

Soviet sources said the order to evacuate some of the estimated 150 Soviet officials and dependents here came late last night and that they would go overland to Damascus since Aeroflot, the Soviet airline, has suspended its flights here as a result of threats against Soviet interests. It could not be learned how many will be evacuated.

The cease-fire agreement was signed in Damascus yesterday by Sheik Saeed Chaaban, leader of the Tawheed militia of the fundamentalist Sunni Islamic Unification Movement, and leaders of the Arab Democratic Party, one of four Syrian-sponsored leftist militias that have been battling Tawheed for control of Tripoli, Lebanon's second-largest city. In 19 days of fighting, more than 500 people have been killed and 1,100 wounded.

The cease-fire was to go into effect at midnight, but there was no immediate indication whether it was being observed. Although all the Syrian-allied militias were expected to follow Assad's orders, it was unclear whether individual units on either side would avoid provocations.

It also was unclear early today whether the Damascus agreement would facilitate the release of the Soviet hostages. Well-informed Soviet sources here reacted with caution to the announcement and said that a day-long absence of statements from the kidnapers concerning the hostages made them nervous.

Chaaban met with Assad and Syrian Vice President Abdul Halim Khaddam, accompanied by an Iranian delegation, in the talks that led to the agreement. Iran, which furnishes Syria with most of its oil needs, is closely linked to Chaaban, as well as to fundamentalist Shiite Moslems.

Syrian television said the cease-fire accord calls for the rival militias to hand over all their heavy weapons to the Syrian Army, which has several thousand troops in the Tripoli area, and for each group to collect light arms from its own men.

Chaaban had previously resisted proposals for the pacification of Tripoli under a plan including the deployment of Syrian troops alongside the Lebanese Army. The Tawheed had insisted that only the paramilitary Lebanese police force should be allowed to remain in the embattled city. The Lebanese police have been largely ineffective in keeping order anywhere in Lebanon in the face of well-armed militia groups.

Private Beirut radio stations associated with the Sunni Moslems said a coordination committee would supervise the evacuation of gunmen and the entry of Syrian troops, police and Lebanese soldiers into Tripoli.

The official Syrian statement of the accord did not say who would be in actual control of the city.

The statement published by An Nahar saying Buckley had been killed was hand delivered to the newspaper office along with a color photograph of Buckley.

"We declare a revenge for the blood of the martyrs by announcing the execution of the death sentence for the American CIA agent in the Middle East and the head of its Lebanon station and the first political adviser at the American Embassy in Lebanon, the spy William Buckley, after publishing this statement," said the group's statement, hand-delivered to An Nahar. All of the other statements the paper has received from Islamic Jihad have proved to be authentic.

While the statement condemned the attack on the Palestine Liberation Organization's headquarters in Tunis, it was also very critical of PLO chairman Yasser Arafat, believed to the the target of the Israeli raid, and Jordanian King Hussein, saying they had "bowed at the door of the U.S."

Buckley, a bachelor from Medford, Mass., became a Foreign Service officer in 1983 after 18 years as a civilian employe of the Army, serving in Washington and Vietnam.

The Islamic Liberation Organization, apparently connected to the Islamic Unification Movement, claimed responsibility in anonymous calls to news agencies for kidnaping the four Soviets -- three diplomats and the embassy doctor.

Wednesday, the body of one of the four, Soviet consular secretary Arkady Katkov, was found in a desolate area south of Beirut following threats by the captors to start killing the Soviet hostages if the offensive against Tripoli was not halted.

Hospital sources said Katkov had been shot three times -- in the head, an armpit and a leg.

Athreat by an anonymous caller to an American news agency Wednesday in the name of the same group to blow up the Soviet Embassy by this afternoon if the fighting was not halted in Tripoli prompted tight security around the mission yesterday and preparations for a possible evacuation.

After three days of conflicting responsibility claims and threats to take the lives of the remaining Soviet captives, the silence yesterday from anyone claiming to speak on behalf of the captors added to the gloom of Soviets. "I am expecting bad news every minute about the kidnaped people," Yuri Suslikov, the Soviet charge d'affaires, told reporters after meeting with Lebanese President Amin Gemayel.

The captors said they seized the Soviets to have Moscow pressure Syria into stopping the assault in Tripoli. The Soviet Union is Syria's main arms supplier and has a friendship and cooperation treaty with it.

Soviet sources said they could not predict how a cease-fire would affect the situation. "I have been here for over three years and I have seen many cease-fires come and go," one said.