Fighting between Syrian and Lebanese forces escalated today in the Bekaa Valley and on the outskirts of Beirut as the Syrian foreign minister held apparently inconclusive talks with Lebanese leaders.

In a bid to relieve Syrian pressure on the besieged Lebanese Christian town of Zahle and its northwestern hills, Christian Lebanese militiamen attacked a Syrian military command post in the summer resort of Chtaura.

State-run Beriut Radio quoted a spokesman from the all-Syrian Arab Deterrent Force as saying that Chtaura had come under artillery fire from the west, an indirect reference to the Christian militias' mountain strongholds about a mile west of the resort. He charged that several houses were damaged by the shelling.

A spokesman for the Christian Phalangist militia denied that any civilian houses had been hit.

A security source in Chtaura, 21 miles east of Beirut, said a hotel previously frequented by Lebanese honeymooners and now a Syrian military communications center received direct hits from the western mountain range running parallel with the Bekaa plain.

This was the first sign that Christian militias have extended their scope for defending Zahle, 30 miles east of Beirut, to Syrian positions behind the frontlines surrounding the central Bekaa market town. Bloody battles around Zahle so far have engaged only Lebanese forces entrenched in hills overlooking the town from the northwest.

The latest fighting has been the heaviest round of clashes since mid-1978, when Syrian peace-keeping forces battled Christian militiamen for control of predominantly Christian East Beirut. Syrian troops had intervened in 1976 to stop fighting in Lebanon between Christian rightist militiamen and an alliance of Palestinians and Lebanese Moslem leftists. The Syrians originally sided with the Christian militias, but that relationship soon deteriorated.

During today's fighting, the two sides battled for control of Zahle's peripheral hills of Hoche all Omara and Al Homar and for the village for Maalaqa, roughly one mile east of Zahle. The private radio station of the Phalangist party interrupted its continuous broadcast of marital music and news flashes to ask blood donor to volunteer at hospitals in Maten, the area northeast of Beirut.

Zahle residents contacted by telephone said the Syrians now were using phosphorous shells. One woman said resident's still lacked bread, water and electricity.

Phalangist radio reported that Syrian forces were using field artillery, rocket launchers and Soviet-made Grad missiles in their onslaught against Zahle and its surroundings.

Meanwhile, Syrian Foreign Minister Abdel-Halim Khaddam concluded talks on the crisis with Lebanese President Elias Sarkis and Foreign Minister Fuad Butros. However, Lebanese political commentators called the meetings "a resounding failure."

In the capital, fighting flared in the southeastern suburbs of Hadath, Kfarshima, Tayyouni and Badaro between elements of the Palestine Liberation Army and Lebanese Army regulars.