UNITED NATIONS, JAN. 5 -- The Soviet Union has promised to put forward a timetable in Geneva next month for the withdrawal of its troops from Afghanistan in less than 12 months, U.N. mediator Diego Cordovez said today.

Soviet leader Mikhail Gorbachev said at a press conference during the Washington summit last month that Soviet troops could be withdrawn in "12 months, maybe less" if outside aid to Afghan rebels fighting the Kabul government were ended. But today's comments by Cordovez are the first indication that the Soviets intend to offer a timetable which, he indicated, would be of short enough duration to satisfy both the United States and Pakistan, where the rebels are based.

{One Soviet official has suggested that the Soviet pullout could begin as early as March, Washington Post staff writer David Ottaway reported. Gorbachev's top Middle East expert, Yevgeny Primakov, said at the time of the summit that he expects to see Soviet troops out of Afghanistan by the end of this year.}

Cordovez also said, however, that Soviet officials have expressed fears that the United States and Pakistan, sensing that the timetable concession is in the offing, may up the ante before agreeing to their half of the bargain -- a cutoff of the flow of arms to the rebels.

The U.N. undersecretary, interviewed by telephone while vacationing in Florida, said that he, too, was worried by recent statements by U.S. and Pakistani officials, including those by Undersecretary of State Michael H. Armacost, that leave open the possibility of new demands beyond the timetable.

Cordovez said he expects to meet Armacost next week, after the American returns from a visit to Pakistan. Cordovez then will travel to Kabul and Islamabad to set up the next round of indirect talks in Geneva between Afghanistan and Pakistan.

On a trip to Moscow in December, Cordovez said, he was told that the Soviet Union wants the next Geneva round to be the last and that a withdrawal time frame of less than 12 months would be offered there. Cordovez would not specify the length of time to be offered, but said he had been assured that it would satisfy the other side.

In addition, he said, the Soviets had dropped their earlier proposal that the timing of a pullout be linked to progress toward an Afghan coalition government in which a local Marxist party, the Peoples' Democratic Party of Afghanistan, would hold a minority position.

Cordovez suggested that the assumption in both Moscow and Washington is that "once they have a time frame, the Afghans are going to be encouraged by everybody to emerge with the broadest possible government, with the least possible ideological content -- a government of personalities."

Cordovez said that during his Moscow visit, Soviet Foreign Minister Eduard Shevardnadze became quite agitated in insisting that there can be no new American or Pakistani demands.

The U.N. official said it would be natural for a feeling of anticlimax to develop after five years of Afghan negotiations in Geneva. This could lead to the fear that something must be wrong and a desire "to be 110 percent sure," he said.

So far, there have been no new demands, only "constructive confusion" in U.S. statements, which may be intended to placate American conservatives, Cordovez said.

He cited a Dec. 21 press conference in which Armacost said that the United States "couldn't stand by and simply observe a withdrawal during which the Soviets attempted to use their residual power to go after the resistance."

Under the draft U.N. accords, the United States would commit itself to end support for the rebels when the Soviet withdrawal begins.

Armacost said Washington would honor that commitment "if we're fully satisfied the agreement will assure a prompt Soviet withdrawal and self-determination for the Afghan people . . . but we have to look at the full agreement, the balance of commitments undertaken, before we are prepared to assume our obligations."