Officials of the Soviet-backed Afghan government have hinted in international talks that the Soviets may be ready to establish a timetable for withdrawing their troops from that war-torn country, diplomatic sources said yesterday.

The hints of a possible withdrawal timetable were reported to be a key feature of last week's session of United Nations-sponsored talks on a political settlement in Afghanistan and were disclosed as President Reagan yesterday marked the sixth anniversary of the Soviet invasion by reiterating U.S. support for the talks and condemning "barbaric methods of waging war" on the part of the Soviet invaders.

Reagan, in a written statement, called a negotiated political settlement "the only reasonable alternative to the bleak prospect of an open-ended military struggle" and said the six-year Soviet occupation of Afghanistan is "an obstacle to overall improvement" in U.S.-Soviet relations.

"Although we welcome any suggestion that the Soviets are prepared to back U.N.-led peace efforts, we will await positive developments on the ground and concrete evidence of Soviet willingness to agree to a timetable for withdrawal of their troops," Reagan added.

Diplomatic sources familiar with the Dec. 16-19 round of U.N.-sponsored talks said representatives of the Soviet-backed Afghan government seemed to express greater willingness than before to discuss specifics of the timetable for a Soviet withdrawal though in the end they refused to put their ideas on the bargaining table.

At one point Afghan Foreign Minister Shah Mohammad Dost reportedly declared that he had details of a withdrawal plan in a folder in front of him on the conference table but that he would discuss them only face-to-face with Pakistani representatives -- and not through the United Nations intermediary, U.N. Under Secretary General Diego Cordovez.

The U.N.-sponsored negotiations are "proximity talks" in which Cordovez shuttles between the two sides. This is because Pakistan, with U.S. backing, will not recognize the Soviet-installed Afghan government.

One source said Cordovez had been "made aware of some thinking" on the Afghan-Soviet side about a withdrawal timetable "over a period of months," and had built on these ideas in making his own proposal for resolution of the conflict to both the Pakistani and Afghan governments.

Among the central issues remaining to be resolved, according to diplomats, is the relationship between withdrawal of Soviet forces and the laying down of arms by Afghan insurgents fighting the Soviet occupation. Pakistan has insisted that the two moves be approximately simultaneous or that the Soviets should withdraw first. The Soviet-Afghan side has insisted that resistance should cease before a Soviet withdrawal.

U.S. officials said it remains far from certain whether the Afghans and Soviets are serious about the possibility of withdrawal or only seeking to blunt worldwide opposition to the invasion and occupation.

"Some fairly hard-headed people have concluded that there is a chance for a political settlement," said a senior State Department official. He said the Reagan administration would like to discuss the question further with the Soviet Union when new U.S.-Soviet talks on substantive issues get under way in preparation for the next summit meeting of Reagan and Soviet leader Mikhail Gorbachev.

Gorbachev's remarks to Reagan in their Geneva summit meeting last month led Reagan to say later there was "evidence" that the Soviets "want a solution to this problem." Administration officials pointed to the Dec. 16-19 round of U.N.-sponsored talks for signs of whether the Soviets are serious.

The currently available reports of what happened in the U.N. discussions, according to U.S. sources, did not provide the clear-cut signs of Soviet seriousness that Washington had hoped for. But they did provide "new signals," a U.S. official said, which keep hopes alive for more substantial movement in the future.