U.S. envoy Philip C. Habib returned to Lebanon today amid indications that his efforts to defuse crisis over missiles in the Middle East center on concrete face-saving moves on the ground here.

Arriving from Israel and Syria at the end of his first week of talks, Habib conferred with Lebanese President Elias Sarkis and Foreign Minister Fuad Boutros at the presidential palace in the hills overlooking Beirut.

The U.S. approach, Western diplomats said, includes efforts to lift the seven-week-old Syrian siege of the Christian city of Zahle in eastern Lebanon.

Habib is expected to travel to Damascus Thursday for fresh talks with Syrian leaders before returning to Israel Friday.

Beirut was relatively quiet as the national carrier, Middle East Airlines, resumed flights from the country's only international airport for the first time in three weeks.

Both incoming and outgoing traffic was light, with many potential passengers preferring not to be guinea pigs in case of a breakdown of the suprise agreement yesterday to reopen the airport.

Syrian troops and their Lebanese allies were reported stepping up their operations along the Mount Lebanon ridge line, especially around the area near the peak of Mount Sanin, overlooking Zahle.

The area has changed hands several times with the Christian militia claiming control 10 days ago.

It was the Syrian use of helicopters in the mountains that led to the present crisis. Israeli jets shot down two of them last month and Syria responded by moving in surface-to-air missiles the next day.

The renewed Syrian operations were seen as an effort to establish control and trade the positions there against concessions elsewhere.

Meanwhile, President Elias Sarkis telephoned Assad in Damascus to express his solidarity with Syria. Normally, the Christian president of this country with a large Christian minority might not have felt obliged to make such a gesture. But the current crisis with the Israel has forced even Assad's archenemy Iraq, to line up with Damascus.

Military specialists reported Syria had ordered a partial mobilization as an official statement announced maneuvers simulating battle conditions involving armored, mechanized infantry and airborne units.

A squadron of Syrian Migs flew just inside the Syrian side of the border with Lebanon, the specialist said, but headed south to north, apparently to avoid prevoking the Israelis.

Meanwhile, more ground troops were reported moved into the border area and the narrow Bekaa Valley strip in which Syria has positioned its Soviet-built missiles.