The United States expressed renewed concern yesterday about a continuing buildup of "combatequipped" Soviet troops in Afghanistan.

State Deapratment spokesman Thomas Reston said the U.S. estimate of Soviet military personnel in that country is now somewhat higher" than 5,000, the figure released by the department last Saturday. However, Reston said reports of 10,000 to 20,000 Russian troops in the country "appear to be exaggerated."

More than 1,000 of the Russians are reported by the State Department to be recently arrived troops equipped for combat. It is this growing contingent, rather than the military advisers and air base guards previously reported in the mountainous country, which is drawing the American puzzlement and concern.

Some U.S. officials have speculated that the Soviets are preparing to shift from an essentially advisory role in the Kabul government's war against rebelling Moslem tribesmen to direct involvement in ground combat. Such a combat role would be highly unusual for the Russians in a Third World conflict.

Soviet pilots and gunners were reported this fall to be flying some helicopter gunship missions against the rebels, and Russians reportely had taken over Bargam air base near Kabul, the capital city, to handle incoming flights.

The officals also said, however, that there is no certainty that the Soviet role is changing, nor do they have an estimate of the numbers of troops which will be sent to Afghanistan in the present round of transport flights. It is still considered possible -- though somewhat less likely as the buildup continues -- that the new Soviet force is intended to protect the airfield and other Russian facilities and personnel, or that the troops are intended to shore up Afghani support for the embattled regime of Hafizullah Amin.

American officials said there is no indication that the Soviet troop influx is connected with the disturbances in Afghaniistan's western neighbor, Iran, or with the U.S. response to Iran's holding of American hostages.