MOSCOW, DEC. 28 (MONDAY) -- The official Soviet news agency Tass reported today that "at least one American adviser" was among those killed in fighting as Afghan troops attempted to open the road to Khost, a garrison town in eastern Afghanistan under prolonged siege by rebel forces.

A top Afghan defense official, speaking at a briefing at the Foreign Ministry in Kabul, said the unidentified American was one of 1,500 casualties counted among the rebels, Tass reported.

Lt. Gen. Mohammed Nabi Azimi, first deputy defense minister, said government troops were about five miles from Khost, contradicting earlier reports on the official Kabul Radio that the Khost road had already been recaptured.

The battle for Khost, a government stronghold 15 miles from Pakistan and at the center of a district long held by the rebels, is one of the largest operations undertaken by Kabul since last January, when the government announced a "national reconciliation" campaign.

President Najibullah, now on a visit in Vietnam, said in a speech Nov. 30 that he would use force to lift the siege. According to the Tass report, government troops moving toward the town outnumbered the rebels by 4 to 1. On Wednesday, Pentagon officials told reporters that large numbers of Soviet troops were engaged in the drive.

Tass said that government troops had seized a large arsenal from retreating rebel forces, including 30 launchers for ground-to-ground missiles and five Stinger antiaircraft missiles. It said the number of rebel troops had increased to 4,000 from 2,000 during the operation and that they were accompanied by "armed Pakistanis and 50 foreign advisers."

The United States has provided aid including weapons to the Afghan rebels, but no reports of U.S. advisers in Afghanistan have received wide credence.

{The State Department could not comment because it no longer maintains a press duty officer on Sundays due to budgetary restrictions, the department's watch officer said.}

Saturday marked the eighth anniversary of the Soviet military intervention, which came as Soviet leader Mikhail Gorbachev has stepped up efforts to find a way to withdraw the 115,000 Soviet troops based there.

The Soviets have offered to shorten the time frame for a withdrawal to 12 months on the condition that the United States and other countries cut off weapon supplies to the rebels.

The fight over the road to Khost has received unusual attention in the Soviet media and was the subject of a special briefing at the Foreign Ministry last week.

In November, Najibullah told the Kabul Grand Assembly that the 40,000 inhabitants of Khost were facing starvation and were being kept alive only by night supply flights.