MOSCOW, JAN. 4 -- Soviet Foreign Minister Eduard Shevardnadze left here today on an unexpected visit to Kabul, raising speculation about a possible Soviet diplomatic move in the eight-year-old war in Afghanistan.

The Soviet news agency Tass gave no details on the trip, which it described as a "working visit." Shevardnadze was last in Afghanistan a year ago, shortly before the Kabul government announced a policy of national reconciliation.

Shevardnadze's trip coincides with a visit to Pakistan by U.S. Undersecretary of State for Political Affairs Michael H. Armacost, who discussed diplomatic efforts for an Afghan settlement with President Mohammed Zia ul-Haq today, Pakistani official media said. It also comes shortly before another round of U.N.-sponsored talks on Afghanistan next month in Geneva.

Soviet Deputy Foreign Minister Yuli Vorontsov told Armacost last October that the next round of talks should be the last, emphasizing a Soviet eagerness to withdraw its troops in Afghanistan -- estimated by the United States to number about 115,000 -- and to find a political settlement to the war.

Last month's meeting between President Reagan and Soviet leader Mikhail Gorbachev produced no visible progress on an Afghan solution, although it topped the summit's agenda of regional issues.

The quickening pace of diplomatic activity in recent weeks seems designed to give impetus to an international resolution of a war that has become a burden for the Soviet Union both internally and externally.

In the past year, the Soviets have reduced the time period they say would be necessary to withdraw the forces. The Soviets now propose a 12-month timetable, only four months longer than the schedule proposed by Pakistan. But Moscow has insisted that a withdrawal should coincide with a halt to all outside interference in Afghanistan, specifically arms supplies from the United States and other countries to the Moslem rebels.

Many western diplomats here say the key issue now is not the withdrawal timetable but the nature of the government left in place in Kabul as Soviet troops withdraw. Some say they have picked up signals that Moscow is ready to consider a future Afghan government that would not be dominated by Najibullah, a former secret police chief installed as Afghan leader in May 1986.

Najibullah promoted the "national reconciliation" policy together with a cease-fire last Jan. 15, in an effort to draw the rebels into a coalition in Kabul, but with the major role reserved for the ruling Communist Party. In the last two years, Moscow has pushed Kabul to adopt more tolerant policies toward opposition and religious leaders.

But as the Soviet press has noted in recent months, the attempt at national reconciliation has not been effective. According to an account by Tass, out of an estimated 5 million refugees, only 110,000 have returned and only 40,000 rebels have laid down their arms. The Afghan guerrillas, or mujaheddin, insist that no significant numbers of Afghans have accepted Kabul's offers of amnesty.

"Despite the obvious successes of the reconciliation policy, its principal goal -- the termination of the war -- has not yet been attained," said Tass in a commentary.

The Tass report claimed, however, that the "limited Soviet military contingent stationed in the country is not an obstacle to a settlement of the Afghanistan stituation," and accused Washington of "fanning" the conflict. The Tass report challenged the United States and Pakistan to join in efforts to end the war and concluded by saying that "the year 1988 may set the beginning of a new era in the history of Afghanistan -- an era of peace and stability."

In recent weeks, the Soviet Union has spotlighted a new offensive by Afghan government troops in breaking a three-month-old siege of the eastern garrison town of Khost, a battle that some say is the biggest winter military confrontation in the war.

The two sides have disputed whether the government troops have opened the critical 80-mile road to Khost, although an independent eyewitness was cited by Reuter this week as saying supplies had reached the town.

{One of the Moslem guerrilla groups fighting in the area conceded today that Soviet and Afghan government troops had lifted the siege of Khost, United Press International reported from Islamabad, Pakistan.}

Soviet Foreign Ministry spokesman Gennadi Gerasimov today told reporters that the situation around Khost is calm. "There are now no military activities in the area of the Gardez-Khost road," he said, adding that 24,000 tons of cargo have been delivered to Khost since Dec. 30.

The battle over Khost, one of the first to receive such detailed publicity from the Soviet side, is seen by some here as an attempt to boost government morale in Kabul and to convince the home audience that any peace talks over Afghanistan are not a symptom of military weakness.