The Palestine Liberation Organization pledged yesterday to support the U.N. pepacekeeping force in its efforts toward the withdrawal of Israeli troops from Lebanon, but it was agreeing to support the cease-fire.

A statement issued by U.N. Secretary General Kurt Waldheim said that PLO leader Yasser Arafat had informed Waldheim of "his acceptance of my call for a general cease-fire."

Arafat, however, made no mention of the cease-fire when he told reporters in Beirut about the outcome of his meeting with MaJ. Gen. Emmanuel Erskine, the commander of the U.N. forcein Lebanon.

"We are offering everything within our means . . . to facilitate the withdrawal of Israeli forces from Lebanon territory," Arafat said.

Another PLO official in Beirut said: "Facilitating the mission of the U.N. in occupied Lebanon is one thing. Stopping raids on Israeli occupation forces is quite another."

In Paris, Abu Iyad, an high PLO official, was quoted by the newspaper Le Matin as saying, "We must return to our bases and positions. If necessary, we will fight against the U.N. troops."

In any case, it was clear that neither Arafat nor any other PLO official could speak for radical Palestine commandos in the field. "Whether diplomat in Beirut, the israeli have broken their own unilateral cease-fire by shelling Palestinian strongholds north of the occupation zone.

In the most significant military move yesterday, Swedish troops of the U.N. force took control of a strategic bridge over the Litani River after an overnight battle in which Israeli soldiers drove off Palestinian guerrillas.

The capture of the Khardali Bridge near Beaufort Castle, an ancient Palestinian-held fortress on a mountain-top, left only one Litani river bridge in Palestinian hands.

Israeli army sources said the battle for the Khardali bridge began when Palestinians ambushed a unit of Israeli soldiers sent in during the night to clear mines from the bridge's approaches.

The fighting intensified into a brief artillery bombardment before the Palestinians withdrew. There were reports in Beirut that eight Palestinians were killed and one Israeli wounded.

Col. Jones Lindgren, commander of the Swedish unit that set up a field post at the bridge, told Israeli radio: "We will hold the bridge and try to prevent anybody getting over it. We want to be kindly, we don't want to shoot."

Asked whether his men would shoot back if fired on, the radio said, Col. Lindgren replied that their orders were to hold the bridge and if his mena had to open fire, they would do so.

Iranian U.N. troops have already set up camp by the Qaaqaiet Bridge in the center at the southern Lebanese front. Only the Abbasiyeh Bridge, near Tyre, is still in Palestinian hands. Attempts by French U.N. forces to take control of that bridge yesterday failed.

Meanwhile, the U.S. Air Force completed the airlift of about 100 tons of equipment for the U.N. forces into Tel Aviv's airport.

The flights, by C-141 and C-5 cargo planes from Italy, carried U.N. stocks of vehicles, communications equipment, office supplies, tents and prefabricated buildings, according to U.S. military officials.