ISLAMABAD, PAKISTAN, DEC. 31 -- A second relief convoy of 184 vehicles arrived in Khost today after the Soviet-backed Afghan government broke the long rebel siege of the eastern garrison town, official Kabul radio said.

The radio, monitored in Islamabad, said the convoy from the provincial capital of Gardez had brought 1,423 tons of food and consumer goods. The first relief convoy reached Khost yesterday.

But spokesmen for Pakistan-based guerrilla parties, which has blockaded the region for more than eight years, said that the siege is continuing and that government and Soviet forces are bogged down about 20 miles from Khost.

Kabul Radio said authorities plan to send 10,000 tons of supplies to Khost in the next five days in daily convoys of 150 to 190 vehicles. It said the supplies brought earlier were distributed among 4,000 people.

"I shed tears of joy when the first convoy arrived in Khost," the radio quoted a Khost resident as saying. "We, the people of Khost, have endured a lot of difficulties and hardships since eight years. We had to buy one sir {15 pounds} of salt for 2,000 afghanis {$40}," he said.

The official Bakhtar news agency said earlier today that Afghan armed forces had to build five bridges to get the first supply convoy of 155 trucks through to Khost. The western-supported rebels fighting the Afghan government and Soviet troops had destroyed and mined about 40 miles of the 80-mile highway from Gardez, it said.

Western diplomats in Islamabad said it was impossible to confirm the exact position as the two sides gave diametrically opposed reports. But, given the strength of the rebels blocking the road and the unfriendly, mountainous terrain, they were surprised at the speed with which the relief force had apparently broken through.

The diplomats said it was possible that the rebels' reports from the Khost area, relayed via radio through the nearby Pakistani town of Miram Shah, might be out of date. "Events may have overtaken the news," one said.