FRANKFURT, West Germany (AP) _ The U.S. ambassador to Lebanon said Wednesday that he is ready to return to Beirut as soon as the safety of his staff can be guaranteed.

Ambassador John McCarthy and members of his staff arrived at the U.S. military's Rhine-Main Air Base outside Frankfurt late Wednesday on a U.S. Air Force C-130 transport plane from Cyprus.

Earlier in the day, he and 29 staffers were airlifted from Beirut, where Christians had threatened to expose the American diplomats to a ''good dose of Christian terrorism'' for what they said was U.S. support of Syria.

At an airport news conference, a weary McCarthy said he left Lebanon ''with great reluctance.''

''It seemed to me that we were doing important work there. The relationship between us and the Lebanese has always been a very warm one.''

But McCarthy said developments in recent days left him little alternative but to close the facility.

''It just seemed to me that in the last several days, it was no longer possible for me to guarantee to Washington that the safety of my staff was secure and it was for that reason that we left,'' McCarthy said.

''But as soon as those questions of security and safety can be resolved, it seems to be that it would be important for us to resume the work that we were doing in Beirut.''

Asked about the fate of 16 foreign hostages in Lebanon, McCarthy said, ''One of my considerable regrets is that I had very little opportunity to make an impact on the hostage situation.

''The hostages are held in west Beirut or in other areas in Lebanon that are basically closed to me and have been for some time.

''We would once in a while pick up the odd rumor, but aside from that, we were not in a good position to do very much for the hostages,'' the ambassador said.

The hostages held in Lebanon included eight Americans. Longest-held is Terry A. Anderson, 41, chief Middle East correspondent for The Associated Press, who was kidnapped March 16, 1985.

McCarthy said he and the embassy staff had worked hard to close the facility when word came to evacuate.

''We spent all last night trying to clear the embassy - about 10 hours - putting things away, destroying things,'' the ambassador told reporters.

About 1,000 Christians massed outside the embassy to protest what they said was U.S. support of Syria. That country entered Lebanon in 1976 under an Arab League peacekeeping mandate but later sided with Moslems in the 14-year-old sectarian civil war.

It is the first time an American ambassador has been pulled from Lebanon since the civil war began. The move at least temporarily ended the American diplomatic presence in the country.

The ambassador and his staff were first flown to Britain's Akrotiri air base in nearby Cyprus. They are expected to leave for the United States on Thursday.