Syrian forces entered the fighting near Lebanon's southern border with Israel for the first time today, pounding an Israeli supported, rightist Christian stronghold with heavy artillery and rocket fire in support of a Palestinian attack, Palestinian and Christian sources here reported.

The surprise Syrian artillery bombardment of Marjayoun carried the possibility of countermeasures by Israel, which has warned against any Syrian intrusion into southern Lebanon.

The Palestinian attack was designed to retake territory lost in months of fighting with Christian rightists in southern Lebanon.

In a fierce midnight-to-dawn battle, the Palestinian forces - reinforced over the weekend by fresh troops drawn from camps near Beirut - recaptured the key crossroads village of Taibe, three miles from the Israeli border.

The Syrian action follows the model set in southern Lebanon by Israel, which for more than six months has been supplying equipment, logistical support and sometimes artillery cover for rightist Christians who have gained control over a string of tiny villages along the border with Israel. The Israeli action was an attemtp to give Israel a narrow buffer zone against Palestinian attacks.

(There was no official Israeli reaction, Washington Post correspondent H.D.S. Greenway reported from Jerusalem. Israeli officials said they are watching developments. Israeli television reported that Israeli television report that Israeli artillery fired across the Lebanese border after several shells landed at Misgav Am, a border kibbutz six miles south of Marjayoun. Twenty Lebanese Christians were reported to have come across the border into Isreal for medical treatment today.)

The use of Syrian artillery and rockets in the two-pronged Palestinian and Christian sources and confirmed by independent Western intelligence sources.

The coordinated Palestinian-Syrian attack on Christian strongpoints followed a secret meeting yesterday between Palestine Liberation Organization leader Yasser Arafat; Zuhair Mohsen, chief of th Syrian-backed Palestinian organization Saiqa, and Naji Jamil, head of the Syrian air force and a close confidant of Syrian President Hafez Assad.

Sources here said the Syria-backed PLO offensive was planned during that meeting, held in the southern Lebanese town on Nabatiyet. In late February, Israel raised a major diplomatic suror that forced the withdrawal of 600 Syrian troops and 10 tanks sent Nabatiyet to control Palestianian heavy artillery that could have bombarded Israel.

Today's coordinated drive publicly signaled the end to a major rift between the PLO, and the Syrians, who as recently as February had ringed Palestinian camps in Beirut with tanks and fought pitched battles with extremist factions of the Palestinian movement.

Now, with the extremists tamed and the mainstream PLO bowing to Syria's will, the two forces are again cooperating.

Truckloads of armed PLO fighters, singing revoutionary songs, were waved through Syrian checkpoints by soldiers who gave them the victory sign, eyewitnesses reported.

According to reports reaching here, Palestinian commandos engaged in house-to-house fighting with Christians in Marjayoun this afternoon after the defenses of the key crossroads town of 15,000 had been softened by the long artillery and rocket barrage.

The Christians gained control of Marjayoun last December. It controls the southern entrance to Fatahland - the rugged slopes of Mount Hermon that the Palestinian commandos use as a staging point for raids into Israel.

The Palestinians need to control Marjayoun so that they can bring supplies in from the southern Lebanon seaport towns of Sidon and Tyre, while the Israeli-supported Christians want it to keep the Palestinians botled up and prevent them from staging raids into Israel. The Christians oppose those raids both because they hate the Palestinians and because the Israelis in the past have staged reprisal raids against Christian villages in southern Lebanon.

The Palestinians depended on surprise to wrest Taibe from rightist Christian forces who captured the town after heavy fighting last Thursday.

By recapturing that village of 8,000, most of whose residents fled when fighting started last week, the Palestinians were bale to get back control of a string of five hamlets running along the Israeli border.

The recapture of Taibe also opened another portion of the east-west supply route to Fatahland and makes it easier on them to defend the beleaguered city of Bent Jbail, until today the only major point still held by the Palestinians along the border between the sea and Mount Hermon. Bent Jbail has been under attack for months by Christian rightists.

The Palestinian counteroffensive, therefore, split the buffer zone that the rightist Christians had created for Israel.

In an editorial today, Ei Baath, the Damascus newspaper published by Syria's ruling Baath Party, said the fighting in the south over the past six months was caused by Israel, which "wants to strike at the Palestinian resistence in south Lebanon and empty the area so as to build a security belt through the threat of force or though the help of special forces.

"We cannot tolerate this situation," the editorial said.

Today's offensive appeared timed to follow the appointment last week of a new army commander whose main job will be to develop an effective Lebanese armed force that can gain control of the south - the only part of the country where fighting has raged since November, when a Syrian-dominated 30,000 man Arab peackeeping force ended a 19-month civil war in this country.

The appointment of the pro-Syrian Victor Khoury as army commander to replace Hanah Siad was opposed strongly by Christian warlord Camile Chamoun, who threatened to escalate the fighting in the south. Most of the Christian fighters in the south are Chamoun supporters and they have been receiving support from Sia's army command.

Now, said a highly placed Lebanese army officer, the Christian fighters in the south are demoralized and angry at everyone - including the supporters in Beirut.

Some observers here saw the Syrian-backed PLO attack as an object lesson to Chamoun - a Syrian way of warning him that he cannot get away with threats against Syrian politics or attempts to obstruct Syrian wishes.

Israeli Foreign Minister Yigal Allon called in U.S. charge D'Aafifres in Jerusalem Monday to warn him of Israel's categoric objection to any sale of U.S. arms to Egypt, Agence France-Presse reported.