Security guards protecting the U.S. ambassador's residence in Beirut fired at a light plane that they thought was about to attack them but that turned out to be a Saudi Arabian civilian craft on its way to Beirut International Airport, U.S. officials said yesterday.

The guards and Lebanese army troops protecting the nearby presidential palace fired .50-caliber machine guns and antiaircraft guns at the plane as it flew low over the buildings, but the aircraft was not hit. One official said he was told that the plane made two passes over the buildings, which are in an eastern suburb of Beirut, before landing at the airport in southern Beirut.

The incident took place at about 5:30 p.m. Tuesday in darkness or near-darkness. Officials said the private Saudi plane strayed from the accepted flight path for approach into the Beirut airport.

Still, officials, who said they have sketchy information about the incident, said they are puzzled about why the Lebanese soldiers did not know that the plane was a "friendly" aircraft cleared to land at the airport. They said they also are not sure why no one was able to hit the plane, although they said they do not know exactly how high or fast it was flying.

One official said the incident demonstrates again how difficult it is to protect against terrorist attack without endangering innocent civilians. They said an airplane would be almost on top of the residence before guards could be certain that it was a hostile craft.

The air space over the presidential palace and the nearby U.S. ambassador's residence has been officially closed to civilian traffic for several weeks, at the urging of U.S. officials in Beirut. Their request came in response to reports that terrorists might seek to attack the residence in small kamikaze planes or in some other way from the air.

The U.S. Embassy, with a sharply reduced staff, has been conducting many official functions from the residence since a suicide truck-bomber demolished an embassy annex northeast of Beirut in September, killing two U.S. military personnel and more than a dozen Lebanese. The original embassy in west Beirut was destroyed in an earlier car-bombing.

The Tuesday incident began when a small, privately owned Saudi plane approached the palace and residence in the eastern suburb of Yarze. Lebanese Armed Forces soldiers guarding the palace began shooting and alerted the residence that a hostile plane was approaching.

"Some of our guys started shooting, too," an official said.

Officials here said they did not know whether U.S. Marines guarding the embassy joined the Lebanese contract guards in firing. They said the guards extinguished all lights along the embassy perimeter so as not to present an easily visible target.

A few minutes after the plane passed, the soldiers "called back to say it was a regularly scheduled flight after all," one official said. "I guess it was in the wrong place."