Beirut — Unless there is a breakthrough in Lebanon soon, there is a strong possibility that Syrian troops will roll into Beirut once again. The Syrians are under increasing pressure to crack the whip over the heads of Lebanon's factions, which continue to fight despite a Syrian-backed cease-fire arranged at last month's peace talks, Eastern and Western diplomats say.
The momentum for Syrian military intervention has grown in the past few days with appeals from the two top opposition leaders, whom Syria supports.
One of them, Druze leader Walid Jumblatt, said, ''The Syrians have intervened in the past, and I think for the sake of Lebanon they should intervene again.''
Although a ranking member of the Shiite Muslim Amal movement said just two weeks ago that Syrian troops would ''never, never'' be allowed in Beirut again, Amal chief Nabih Berri said over the weekend, ''I think it's necessary for the (Syrian) Arab Deterrent Force to return in order to prevent partition.''
Ironically, all Lebanese factions said they were pleased when the Syrians were forced out in August 1982 under an American-sponsored plan for their evacuation along with Palestinian guerrillas, after being thrashed by the Israeli invasion. Their pullback to positions in the Bekaa Valley marked the end of a controversial six-year presence as peacekeepers under an Arab League mandate. During their stay the Syrians made no headway in preventing civil strife.
Diplomats say the Syrians are not enthusiastic about the option. But President Hafez Assad is reported to be deeply frustrated by the inability of the Christians, Muslims, and Druzes to end the war or reach an agreement for power-sharing in the government. As Lebanon's turmoil increases, Syria's credibility decreases.
Syria's main motive for sending in troops would be to quell the fighting and promote an atmosphere conducive to dialogue on reforms. Syrian-backed Muslim groups have been either unable or unwilling to fully control west Beirut since they took it over in early February.
Incidents against diplomats continue. Despite the release Sunday of American professor Frank Regier and French engineer Christian Joubert after Amal's intervention, two kidnapped Americans are still missing. Seven Soviet Embassy vehicles have been stolen at gunpoint since mid-February. Three cars have been heisted from US Embassy personnel recently. Norwegian, Yugoslav, Italian, Greek, and United Nations envoys have all reported similar cases, often including robbery of funds and documents.
Another cause for alarm is the emergence in west Beirut of the ''Hizbollah'' militia along key points of the ''green line'' that divides the city. This is the first time the radical group has been seen working openly in the capital. The Islamic fundamentalist Hizbollah, or ''Party of God,'' is trained, armed, and funded by Iranian Revolutionary Guards. Western diplomats say the group may have had a role in the bombing of US and French multinational force facilities last year.
Various estimates put their strength in Beirut at between 600 and 2,000. There are other, smaller extremist Muslim groups which the two major militias, Amal and the Druze Progressive Socialist Party, have not drawn into line.
There is growing speculation that Amal, the largest militia, has so far chosen not to move on the radical Shiites because such a move could prove costly by causing dissension among Muslim ranks, especially the younger fighters who have had dual loyalties in the past. Lacking political progress, Amal's Berri is in particular danger of losing support, since he has little to show for his moderation.
Lebanese and East-bloc sources with strong Syrian ties claim that Damascus would require an official invitation from President Amin Gemayel to send in troops. They suggested it would specify a time limit, such as six months, the period allocated at the peace talks for coming up with a new constitution. Such a time limit would cater to Christian fears of Syrian attempts to absorb Lebanon into the historical ''Greater Syria.''
Sources from the Christian Lebanese Forces militia say it is understood that the Syrians, if summoned, would be assigned to Muslim-dominated west Beirut only , and perhaps the nearby mountain town of Souk al Gharb. They noted that President Gemayel and his father, Pierre, leader of the Christian Phalange Party , would be in charge of preventing provocations in Christian east Beirut.
East-bloc sources say Syria would probably have to send in roughly 4,000 to 5 ,000 men. There are repeated but unconfirmed reports that troops in the Bekaa have begun to paint their tanks and helmets white, the color used by peacekeeping forces to show neutrality.
There were some hopeful signs Tuesday that the Lebanese government was coming closer to finding both a reform package and a security solution that would preclude the necessity of a Syrian presence. For several days there have been daily shuttles by Lebanese presidential advisers and opposition figures between Beirut and Damascus to consult with the Syrians on possible solutions. However, the anticipated summit between Gemayel and Assad has been postponed because Damascus wants specific programs and results on the ground before receiving Gemayel again.
The main snag continues to be a chicken-and-egg dispute over whether military or political steps should be taken first. ''No cease-fire could hold and last unless it is based on a political solution,'' Berri said over the weekend.
Christians, however, have argued there must be an end to military pressure - the only leverage the majority Muslims have in the Christian-led state - before power-sharing disputes can be settled.