ISLAMABAD, PAKISTAN, DEC. 30 -- The Soviet-backed Afghan government said a relief convoy arrived in Khost today, but the guerrillas besieging the southeastern town for nearly three months called the claim "a total lie."

Kabul radio said thousands of people turned out to welcome the soldiers and officers guarding the convoy, showering them with flowers and dancing in the streets.

Western-backed rebels in Pakistan denied the report. A spokesman for rebel alliance leader Younis Khalis said Soviet and Afghan government forces were still bogged down 20 miles short of Khost.

"It's a total lie," said Hamed Gailani, deputy leader of one of the seven rebel parties. "Even if it {the convoy} did arrive, it's good for the mujaheddin because they will have good supplies" to capture.

{In Washington, U.S. officials said they had no confirmation of the claim but would not be surprised if a convoy had reached Khost.}

Kabul radio said 155 trucks brought 1,200 tons of food and consumer goods to the eastern town from Gardez, the provincial capital, this afternoon. The radio said another convoy of 150 vehicles with 1,600 tons of supplies left Gardez today for Khost.

The mujaheddin guerrillas had maintained up to this morning that the 80-mile Gardez-Khost highway was still blocked by their fighters. The anti-Soviet rebels had held the road since 1979 but only isolated the garrison town near the Pakistani border about three months ago.

Thousands of Soviet and Afghan government troops launched a major drive earlier this month to clear the way through the narrow, winding valley. Thousands of turbaned tribesmen fought back in what the rebels say was one of the biggest battles of the nine-year-old Afghan guerrilla war.

The Soviet news agency Tass said the rebels lost up to 2,000 men killed and had to abandon their main base in the area, Saranay. The mujaheddin had put their casualty toll at less than 100.

Afghan President Najibullah said at a constitutional assembly in Kabul last month that the 40,000 people of Khost faced starvation and deprivation because of the siege.

Kabul radio said Sulaiman Laeq, a member of the ruling Politburo, and Deputy Defense Minister Nabi Azimi, who commanded the government forces, accompanied the convoy to Khost.

Afghan exiles in Pakistan have said that even if the government did manage to break the siege it would not be able to keep open the road, which winds through steep valleys in snow-capped mountains.

The local Jadrani tribesmen, under guerrilla commander Jalaluddin Haqqani, have controlled the area for more than eight years and have easy access to supplies of ground-to-ground rockets, heavy machine guns and even antiaircraft missiles from across the border.

The reported lifting of the siege came shortly before U.S. Undersecretary of State Michael Armacost was due in Pakistan for talks on an Afghan peace settlement. U.N. mediator Diego Cordovez is also expected in Islamabad later in January, fresh from talks in Moscow.