The United States expressed concern yesterday that the Soviet Union may be in the process of changing Afghanistan from an autonomous buffer state to a Moscow-dominated client, with serious repercussions in the already unstable region of southwest Asia.

In an unusual Saturday briefing, a senior State Department official detailed recent troop movements on the Russian side of its Afghanistan border as well as less extensive -- but more alarming -- movements of Soviet combat troops into Afghanistan itself.

In the two weeks since the military shifts have become clear, the United States has expressed concern to the Russians at high levels on three or four occasions, the official said. There was no report of the Soviet response.

Afghanistan's neighbors, Iran on the west and Pakistan on the east, are reported to be concerned about the increasing Soviet military role, and some concern is also reported in India, another power in the region.

Officials said yesterday as in the past that no direct link is evident between the Soviet moves in Afghanistan and the nearby crisis between the United States and Iran. But some officials find it paradoxical that the United States is under heavy fire in parts of the Islamic world for its presures on Iran in the hostage crisis, while the Soviet Union's military role in crushing a rebellion by Islamic tribesmen in Afghanistan is little noted and relatively uncontroversial.

By calling attention to the Russian activities, the United States appears to be seeking to increase their cost to Moscow in world opinion, and Islamic world opinion especially. Little hope was expressed yesterday that U.S. objections could stop the Soviet movements, nor was there any threat of physical action by the United States.

According to the State Department official, who did not permit use of his name, the equivalent of three "combat-equipped" Soviet battalions arrived at Bargam Air Base north of Kabul, the Afghan capital, in the past two weeks. There is no evidence that this Soviet force of about 1,500 men has undertaken actual military operations against the rebel tribesmen who are threatening the Afghan regime, the official said.

There was no charge that the Soviets have invaded Afghanistan, since the troops apparently were invited by the pro-Soviet regime of Hafizullah Amin. The historically friendly ties of the Soviet Union and Afghanistan have become much closer since the April 1978 coup that placed the present ruling group in power. Three months ago. Amin, a former student at Columbia University teachers college in New York, ousted Nur Mohammed Taraki, the original coup leader.

On the Soviet side of the Afghan border, two Russian divisions already stationed there have been brought up to full strength and have moved out of their usual garrisons, the State Department official said. In addition, a third force of division strength, made up of elements of several other units, has moved into the area, the official added.

A Soviet headquarters unit for the Afghanistan border force has also been recently identified, reporters were informed.

There is no clear sign of the plans or purposes of the Soviet elements in or near Afghanistan. The force as presently described is insufficient to carry on a nationwide Afghan offensive against the rebel tribesmen, according to the official. But he said that it would be large enough to secure key areas and lines of communication in the mountainous country.

Afghanistan's own army has dwindled -- from 90,000 to 100,000 men a year ago to about 50,000 today, according to the briefing. This was said to be due to desertions, casualties and inability of the ruling regime to win popular support. The embattled regime's ability to manage the situation was said to be steadily deteriorating.

The State Department official said the United States has a "hands off" policy with regard to the insurgency of Moslem tribesmen. The rebellion was described as locally based, widespread and "remarkably spontaneous." According to the official, the tribesmen are seeking to preserve their traditional way of life against a brutally repressive government.