The Reagan administration moved quickly yesterday to fend off an attempt in the Senate Foreign Relations Committee to apply the War Powers Resolution to U.S. troops in Lebanon, but it agreed to seek congressional approval if it decides to expand the number or role of U.S. peace-keeping forces there.

The 1973 War Powers Resolution, passed in reaction to the unauthorized commitment of U.S. troops to Vietnam, requires the president to seek congressional approval to keep U.S. troops in hostile situations for more than 90 days.

Sen. Claiborne Pell (D-R.I.), ranking minority member on the committee, said yesterday he would have had the votes to apply the resolution to the 1,200 Marines now in Lebanon.

However, Undersecretary of State Kenneth W. Dam, in a letter to the committee, said that it would be "highly premature and unwise" to apply the resolution to Lebanon because it would "amount to a public finding that U.S. forces will be exposed to an imminent risk of involvement in hostilities."

Such a finding, Dam added, "could give entirely the wrong public impression as to what results these negotiations over the withdrawal of foreign forces from Lebanon are intended to produce."

Dam said the administration would "seek authorization from Congress as soon as possible" for any further commitment of U.S. forces after the completion of negotiations over the withdrawal of foreign forces from Lebanon and the establishment of a permanent peace-keeping force.

An amendment approved by the Senate committee yesterday states that the president must obtain statutory authorization for "any substantial expansion in the number or role of the U.S. armed forces in Lebanon or for the creation of a new, expanded or extended multinational peace-keeping force in Lebanon."

Committee members said this would allow the president to send additional troops without waiting for Congress to complete action on an authorization request.

The amendment leaves the door open for Congress to apply the War Powers Resolution later.

Lebanon has asked the United States, France and Italy to double the size of the 4,500-man peace-keeping force, even as negotiations are proceeding on the withdrawal of all foreign troops from Lebanon and the establishment of a permanent multinational peace-keeping force.

The Senate amendment was attached to a $251 million emergency economic and military aid package for Lebanon, which was approved by the House Foreign Affairs Committee on Tuesday. However, the House amendment does not address potential increases in the temporary peace-keeping force in Lebanon.