A senior State Department official, apparently responding to conservative Republican criticism that the department may "sell out" the Afghan resistance, said yesterday the administration wants to see details of a Soviet timetable for troop withdrawal from Afghanistan before deciding whether to end U.S. military aid to the rebels.

"The devil's often in the details in these matters and we'll obviously want to look at that full range of details before we make our judgment that we're prepared to honor obligations that are embodied in the Geneva understandings," Undersecretary of State Michael H. Armacost said.

"We'll have to look at the full {Geneva} agreement, the balance of commitments undertaken, before we are prepared to assume our obligations," he added.

But Armacost reaffirmed the U.S. commitment to live up to the Afghan peace accords being negotiated in Geneva between Pakistan and the Soviet-backed Kabul government "if we're fully satisfied that the agreement will assure a prompt Soviet withdrawal and self-determination for the Afghan people."

Until then, U.S. support to the Afghan resistance, now running above $600 million a year, will continue, Armacost said.

He refused to say whether the administration would accept a 12-month timetable for the withdrawal of an estimated 115,000 Soviet troops, as Soviet leader Mikhail Gorbachev has proposed, provided U.S. aid to the Afghan rebels is halted. Armacost said the administration believes a withdrawal could be carried out "much more promptly" than that and "well before the end of next year."

His statements seemed calculated to bridge recent conflicting declarations from the White House and State Department over the terms of ending U.S. military assistance.

Armacost's wording seemed to add a significant qualification to the U.S. commitment to respect the Geneva peace accords, which call for an end to all outside interference once a Soviet withdrawal begins. The administration has already pledged to be a guarantor of the accords together with the Soviet Union once they are completed and signed.

Armacost made his comments during a briefing on the department's annual report on the political and military situation inside Afghanistan in the eight years since the December 1979 Soviet invasion.

He said military, political and diplomatic trends had changed "in significant ways" in favor of the U.S.-backed resistance over the past year. There has been "a dramatic improvement" in the rebels' ability to counter Soviet forces, which Armacost said had adopted "a more reactive and somewhat more defensive posture."

The Soviet-backed Kabul government's army continued to suffer high desertion rates and low morale while its leader, Najibullah, in trying to concentrate power in his hands, has exacerbated factionalism within the ruling Communist Party.