Syrian troops of the Arab deterrent force mounted a two-pronged tank on the embattled Christian town of Zahle in the Bekaa Valley, a spokesman for rightist Lebanese forces said today.

The spokesman said the assault was aimed at capturing bills that overlook Zahle, 30 miles east of Beirut.

"If the Syrians succeed in taking over these hills, Zahle will be completely surrounded," the spokesman for the forces, under the command of Phalangist militia leader Bashir Gemayel, said.

The spokesman said a "defensive attack" was holding off the Syrian troops and Christian militias destroyed two Syrian tanks in the late afternoon fighting.

A commentary on Radio Damascus today strongly condemned the Lebanese militia fighting Syrian troops and criticized a document issued this week by the Lebanese Front, an alliance of Lebanese Christian leaders including Phalange Party head Pierre Gemayel and National Liberal Party leader Camille Chamoun.

The statement, called by the Syrian commentator a "black document," called for Syrian withdrawal from Lebanon and rejected settling Palestinians in southern Lebanon.

Chamoun said today that the Syrians should pull out of Lebanon and be replaced by U.N. forces. About half of the 30,000 Syrian troops dispatched to Lebanon in November 1977 as part of an Arab League-sponsored peacekeeping force are stationed in the Bekaa Valley.

Meanwhile, Beirut radio and the official Lebanese national news agency said an Israeli force entered Lebanon today and blew up a house in Haddatha, a few miles north of the border, injuring three civilians.

[In Tel Aviv, a military source said the raid "did not occur at all and, if it did, we have no reason to hide it," United Press International reported.]

Haddatha is under the control of the Irish contingent serving with the U.N. force that was deployed in Lebanon following the Israeli invasion in 1978.

News services reported the following:

The Palestinian news agency WAFA said in Damascus that Palestinian commandos inflicted heavy losses on Israeli forces in a 90-minute clash in western Galilee yesterday while losing five of the commandos.

An Israeli military spokesman disputed the WAFA account, saying an Israeli patrol had spotted five guerrillas in southern Lebanon, chased them across the border into Israel and killed them, while suffering no Israeli casualties.

Meanwhile, a bomb was thrown into the grounds of the French Embassy in Beirut today. In a separate attack a car belonging to the embassy was blown up. There were no reports of casualties.