U.S. officials have detected fresh evidence in the past two days of a possible attack against the U.S. ambassador's residence in Lebanon and have asked Lebanese officials to curtail all flights around the residence to help preclude an attack from the air, administration officials said yesterday.

The U.S. Embassy in Beirut has reduced its staff to about nine professional employes and between 25 and 30 military personnel, according to the officials. The State Department has declined to provide a specific number, in part for security reasons and in part because of Leb-anese concerns that Washington will withdraw altogether. The remaining staff has been working mostly out of the ambassador's residence since a suicide truck-bomber demolished the principal embassy offices east of Beirut on Sept. 20. The residence is in the suburb of Yarze, several miles east of the city center and near the presidential palace.

Security officials believe that they have seen vehicles attempting to "probe" the perimeter around the residence to test reaction times and look for weaknesses in the defenses, officials in Washington said.

They said that the probes in the past two days follow a series of threats and warnings that have been received since the Sept. 20 bombing.

Intelligence sources also have said that some of the explosives brought into Lebanon for the Sept. 20 attack were not used and remain in the area.

They have said that they fear another attack before the Nov. 6 U.S. election.

Several of the warnings within the past two weeks have suggested the possibility of an attack from the air, officials said yesterday. Security officials considered a number of measures to guard against such an attack, including shoulder-fired Stinger missiles, they said.

Most of those measures were rejected as impractical because they would require the presence of more U.S. troops and because it would be difficult to distinguish a suicide-plane or other attacking aircraft from a harmless civilian craft until the last moment.

This was particularly a problem because of helicopter traffic in and out of the presidential palace, administration officials said.

After failing to achieve a reliable schedule for that air traffic, U.S. officials asked the Lebanese government to curtail all flights in the area as much as possible for the next two weeks or so, officials said.

Some Lebanese troops armed with antiaircraft guns also have been deployed in the area, they said.

In addition, U.S. security officials have stationed observers where they might see an aircraft coming and give some warning, the officials said.