Palestinian forces fired artillery barrages into Israel today in an attempt to cut the flow of supplies to Israeli-backed rightist Christians fighting to retake this strategic crossroads village in southern Lebanon.

A Palestinian commando captain said his guns were firing over the Israeli border, three miles away, as part of the daylong battle for control here. At nightfall, the Palestinians still held the village, although observers said rightist gunners had fired more than 200 heavy artillery shells into Taibe.

"We are shelling the roads inside Israel leading to the Christian territory. We are trying to shut off their supply routes," said the Palestinian officer, standing beside his command car and ordering artillery barrages over a portable radio.

There were no reports of Israeli retaliatory fire, although yesterday Israel admitted firing into Lebanon when artillery barrages hit a border kibbutz.

(The Israeli government said it has received no confirmation of the reports of Syrian involvement in the border area, Washington post correspondent H.D.S. Greenway reported from Jerusalem. Officials would only say that, if the reports were true, they would present Israel with "a new situation." The officials said they are "watching developments closely."

The Christians, who until this week had almost completed a narrow buffer zone along the 60-mile-long border with Israel, clearly were losing ground to the new, Syrian-backed Palestinian offensive.

The dawn attack to retake Taibe, which fell into Palestinian hands yesterday, failed and Palestinian sources said the rightists withdrew in disarray.

According to the Palestinians, the Christians left three disabled armored personnel carriers filled with troops on the battle field just 65 yards away from the Palestinian positions. The rightists encircled the vehicles with artillery fire to prevent the Palestinians from reaching them. The Palestinians said they refrained from firing armor-piercing, rocket-propelled grenades at the vehicles in hopes of capturing the personnel. They said they could hear the occupants of the armored personnel carriers calling for help in Hebrew.

The shelling was so heavy in the Taibe area that palestinians refused to allow correspondents close to the battlefield.

The renewed intensity of the fighting in southern Lebanon - the only part of the country where battles have raged since November when a 30,000 man, Syrian-dominated Arab peace force ended the 19-month civil war - carries the threat of spreading into a wider Middle East War involving Syria and Israel.

Israel has supported the rightist Christians for the past six months as the Christians captured a string of villages from the Mediterranean Sea to Mount Hermon to create a buffer zone along the Israel border.

Syria entered the fighting for the first time yesterday, using its heavy artillery and rockets to pound the military garrison of Marjayoun held by the Christians There were no further reports of Syrian artillery fire today.

Israel, which reacted with strong threats in February when Syria moved 600 troops and 10 tanks to the southern Lebanese town of Nabatiyah in an attempt to prevent Palestinian artillery from shelling the Jewish state reacted more mildly today.

The infulential Tel Aviv newspaper Maariv urged the Israeli government "to clarify quickly whether Damascus is changing partners on the Lebanese battlefield, because in that case we will have to reconsider our policy."

According to an Associated Press report from Tel Aviv, the newspaper said Israel had tolerated Syria's presence in Lebanon only as long as Syria sided with the Christians against the Palestinians.

The sudden Syrian alliance with the Palestinian forces, whom they had been fighting in camps in Beirut as recently as February, also appeared to bewilder the rightist Christian leaders in Beirut.

Since last June, Syria had fought in the civil war on the side of the Christians against a Palestinian Moslem alliance. Since they changed hats after the October cease-fire to become the mainstay of the Arab peace force, they have concentrated on taming the extremist elements of the Palestine Liberation Organization.

Having apparently achieved that aim, the Syrians have now joined forces with the Palestinians in the south against the Christians.

According to knowledgeable diplomatic experts in Beirut, the key to the Syrian involvement in yesterdays heavy shelling of Marjayoun was the use of 140 mm rockets - A type of weapon no Palestinian faction has in its arsenal.

Informed sources in Beirut said the Syrians fired their artillery into Southern Lebanon from positions in the Bekaa Valley and from the town of jezzine.

U.S. Ambassador Richard parker and Foreign and Defense Minister Faud Butros met today in Beirut for what was believed to be a discussion of how to meet the danger posed by Syrian and Israeli involvement in the border conflict. The United States has been acting as an intermediary between Lebanese, Syrian and Israeli officials over events in the border region.