Senior administration officials have moved quickly to reaffirm a U.S. commitment to stop supplying arms to rebels in Afghanistan once the Soviet Union begins a troop withdrawal, apparently in an attempt to clear up confusion caused by President Reagan's comments suggesting there would be no such cutoff.

The reassurances were conveyed to the Soviets during the summit meeting here between Reagan and Soviet leader Mikhail Gorbachev, according to Undersecretary of State Michael H. Armacost.

"We made clear that we will stand by the obligations that are embedded in the Geneva understandings once there is agreement on the timetable for withdrawal," Armacost said Friday in a briefing for Washington-based foreign correspondents.

Assistant Secretary of State Richard W. Murphy and several other senior administration officials have made similar statements reconfirming the U.S. commitments, which include a pledge of noninterference in Afghanistan's internal affairs -- understood by all sides to include an end of military supplies to the Afghan anticommunist resistance by Pakistan, Saudi Arabia and particularly the United States, which is providing more than $600 million annually.

The "understandings" are contained in four "instruments" on an Afghanistan settlement being negotiated in Geneva between Pakistan and the Soviet-backed Kabul government by U.N. mediator Diego Cordovez. The accords provide for a simultaneous start on a Soviet troop withdrawal and an end to outside interference 60 days after they are signed.

Reagan's remarks on Afghanistan in an interview with American television network anchors just before the summit caused considerable confusion about whether the United States was changing its policy.

Reagan summarily rejected the idea of even a temporary U.S. cutoff of arms to the Afghan resistance if the Soviets made a commitment to a 12-month withdrawal timetable.

"I don't think we could do anything of that kind because the puppet government that has been left there has a military and it would be the same as what I'm arguing about with regard to the freedom fighters in Nicaragua," Reagan said. "You can't suddenly disarm them and leave them prey to the other government."

Gorbachev, at his news conference Thursday, said the Soviet Union would name "the beginning of the withdrawal of Soviet forces" but that "this must at the same time become the start, the beginning of an end, to arms and financial supplies to the insurgent forces."

Despite the lack of any summit agreement on a Soviet withdrawal timetable, Gorbachev provided further details on the Soviet position, which U.S. officials found of considerable interest.

Linking the start of a Soviet withdrawal to a similar start of an end to U.S. aid to the resistance, Gorbachev said that "from the very first day this is declared," Soviet troops "will stop taking part in military operations" and "all military action" will cease.

This was the first time Gorbachev, or any other Soviet official, had said this.

The Soviet leader also formally reaffirmed what Afghan President Najibullah had announced Nov. 30 -- that Soviet troops could be withdrawn in 12 months or less. But he fixed no firm date.

Gorbachev shed light, however, on the Soviet view of how the thorny question of an interim coalition government should be resolved, again breaking new ground. He said resolution of this issue could start after, rather than before, a Soviet withdrawal began and gave no indication that it is still a condition for a pullout.

Furthermore, he said formation of an interim government should be left up to "the actual various forces within Afghanistan itself -- all of the sides concerned."

Armacost said that, on the basis of these and other Soviet remarks during the summit, he had concluded that "the Soviets do not link the withdrawal of their troops to prior resolution of issues of an interim government or transitional arrangements among Afghans." He added, "Both sides agreed that those are issues to be resolved by the Afghans themselves."

The Soviets have been backing Najibullah in his effort to set up a coalition government dominated by the Afghan Communist Party. Administration officials have rejected such a government and insisted that Afghan resistance leaders play "a central role."

Both sides seem to agree now that U.N. mediator Cordovez should accelerate his efforts to find a formula for the makeup of an interim coalition government acceptable to all parties.

Earlier this month, Cordovez flew to Geneva and Rome for secret talks with the exiled Afghan king, Mohammed Zahir Shah, Afghan resistance representatives and other independent Afghan political figures.

The U.N. mediator refused in a telephone interview to discuss his talks, but denied they had anything to do with his other efforts to reach an agreement on a short Soviet withdrawal timetable. Diplomatic sources said, however, that Cordovez was working on the formation of a tripartite interim government along the lines proposed by Pakistan President Zia ul-Haq.

Today, Cordovez is scheduled to go to Moscow for talks with Soviet officials in connection with the next round of Geneva talks between Pakistan and the Kabul government, which Gorbachev has suggested could be the last one before an agreement is reached. The talks are now expected to take place in late January or February.

Gennadi Gerasimov, Soviet Foreign Ministry spokesman, said yesterday that conditions for Soviet withdrawal from Afghanistan are "not very big ones."

Speaking on ABC News' "This Week With David Brinkley," Gerasimov said, "We are going to withdraw, and the process of national reconciliation will take place, and you will have a nonaligned Afghanistan; nonsocialist, also." Staff writer Judith Havemann contributed to this report.