Israel shot down a Syrian Mig25 reconnaissance jet just east of Beirut today as a controversy over heavy weapons left behind by evacuating Palestinian fighters threatened to halt a promised Israeli withdrawal from Beirut's airport.

The Syrian jet, a sophisticated, Soviet-built fighter, was shot out of the sky early this morning while the Syrian Army was withdrawing the last of its regular troops from Beirut. The departure of the Syrians, who left in an overland evacuation through the Israeli-controlled hills east of the Lebanese capital, and of another two boatloads of Palestine Liberation Organization fighters leaves only about 700 Palestinians still to be evacuated early Wednesday morning, according to PLO sources.

Despite the successful near-completion of the evacuation, Israeli officials have said they will not pull back their forces from the Lebanese capital until heavy arms left behind by the PLO are turned over to the Lebanese Army in accord with the evacuation agreement, official Lebanese sources reported. The Israeli pullout is the second phase of the accord, arranged by U.S. special envoy Philip C. Habib during two months of negotiations.

It was not immediately clear how the high-flying jet was brought down. The plane crashed into the top floor of an apparently empty five-story building in Rabiyah, four miles northeast of Beirut.

The completion of the PLO evacuation is gradually shifting attention to the continued presence of the Israeli and Syrian forces in Lebanon. There was speculation in Jerusalem that the Syrian plane was attempting to learn the disposition of Israeli forces east of Beirut as the crisis in Lebanon enters a new stage in which Israel has vowed to seek a Syrian withdrawal through diplomatic means, Washington Post correspondent Edward Walsh reported.

A terse announcement by the Israeli military command said the jet was shot down while on reconnaissance, adding that Israeli planes returned unharmed. "We do not allow any Syrian planes to cross the cease-fire lines and fly over our forces," the military spokesman's office said.

Later, Israeli sources said surface-to-air missiles had been used to hit the jet, Walsh reported. Israel is known to have Hawk missiles, but it had not been reported to have them stationed in Lebanon.

The Christian rightist radio, The Voice of Lebanon, reported that Israeli Navy gunboats cruising off the port of Beirut had fired on the plane.

Lebanese military sources said one crewman of the jet had bailed out and apparently was captured by Israeli soldiers on the ground. The body of the other was found charred in the jet's wreckage. Police said no civilians were injured in the crash.

The incident came as U.S. special envoy Philip C. Habib met with Lebanese Prime Minister Shafiq Wazzan to discuss an Israeli ultimatum over the disposition of the heavy weapons the PLO left behind with its Lebanese Moslem allies in West Beirut.

According to the PLO evacuation agreement negotiated by Habib to bring an end to Israel's siege of West Beirut, the PLO was to turn over all its heavy weapons--tanks, artillery, antiaircraft batteries and mobile rocket launchers--to the Lebanese Army. Instead, as the PLO withdrawal by land and sea neared an end, these weapons have turned up with local militia groups, such as the Morabitoun.

Last weekend, Israeli chief of staff Lt. Gen. Rafael Eitan said the Morabitoun, a force of 1,500 armed Beirut Arab nationalists, must not be allowed to remain in West Beirut. Official Lebanese sources added today that Israel had informed the Lebanese government that it wanted the Morabitoun and other such forces in West Beirut disarmed and the heavy weapons they got from the PLO turned over to the Army as was required by the evacuation agreement.

If that is not done, according to these Lebanese sources, Israel said it would not withdraw its forces from the airport, the Beirut port area or from behind key crossing points between Christian East and Moslem West Beirut. Those crossing points are now in the hands of French and Italian troops from a 2,100-man peace-keeping force.

Israel also has requested that it be allowed to maintain its own air controllers at the Beirut airport when and if its troops withdraw, said Lebanese government sources. The Israelis also asked that one of the airport's two runways be reserved for Israeli military planes. Both requests reportedly were rejected by the government as unreasonable and not part of the Habib agreement.

The opening of Beirut International Airport, closed since the second week in June by the Israeli invasion, has been considered a key to normalizing conditions in the capital after Israel's destructive 10-week siege of its Moslem western sector.

According to the Habib plan, the airport was supposed to be taken over by the Italian troops of the multinational force as the Israelis withdrew. Lebanese officials had expected this withdrawal to take place early this week and had predicted the airport might be opening as early as Thursday.

Israel has now refused to allow the Italians to move into the airport area until the issue of the heavy arms is resolved.

After today's meeting between Wazzan and Habib, the Lebanese leader said they agreed that the issue of the Morabitoun's presence in Beirut was an internal Lebanese matter that was not an issue dealt with in the Habib accord and should be of no concern to Israel.

The prime minister said, however, he would continue to hold talks with West Beirut Moslem and leftist leaders to try to find some solution to the question of the PLO heavy arms. It is unclear, however, if the political leaders can induce their military commanders to give up the arms.

The PLO weapons have become something of a status symbol among the dozen leading militia groups. The Morabitoun, for instance, has lined up some of its new antiaircraft guns and armored cars around their headquarters in the Gamal Abdel Nasser Mosque on the Corniche Mazraa Boulevard.

Ibrahim Koleilat, the Morabitoun's leader, publicly denies that the heavy weapons his group is displaying are those turned over to him by the PLO. But with only one last boatload of PLO fighters still to leave Beirut, none of their heavy weapons have been turned over to the Lebanese Army, which the leftist militias suspect of being a tool of their longstanding opponents in Lebanon's Christian rightist community.

Koleilat and other militia leaders insist that as long as there is a threat that they might be attacked by the 20,000-strong Christian militia of President-elect Bashir Gemayel and its de facto Israeli allies, they will not give up their weapons.

The controversy has come to a head as the evacuations are on the verge of conclusion. Today the last overland evacuation along the Beirut-to-Damascus highway was completed with the withdrawal by Syria of the final 1,000 troops from its 85th Brigade. The Syrians came to Beirut in 1976 as part of the Arab Deterrent Force that the Arab League hoped would be able to keep peace after the 18-month civil war.

The Syrians departing today and the 1,200 who left yesterday drove to Lebanon's Bekaa Valley to be deployed there with the remaining 25,000 Syrian soldiers of the Arab Deterrent Force.

Syria has said it will not withdraw its forces from the Bekaa Valley and northern Lebanon, unless the Lebanese government and the Arab League formally request the departure. Israel has said it would not withdraw its 80,000 troops occupying southern Lebanon and the outskirts of Beirut until the Syrians withdraw.

Today's departure of 841 PLO fighters on their way to new homes in North Yemen meant that more than 10,600 PLO members and soldiers of the Palestine Liberation Army have left Lebanon. A total of 2,500 Syrians also left along the overland route in the past two days.