ISLAMABAD, PAKISTAN, JAN. 5 -- Soviet-led forces used a major flanking maneuver to avoid a murderous mountain pass and breach the Afghan rebel stranglehold on the garrison town of Khost, according to western diplomats who have been following the unusual wintertime battle in Afghanistan's southeastern Paktia Province.

Western diplomats today confirmed earlier reports that at least some Soviet and Afghan Army units had broken out of the mountains to the west of the Khost plain to bring a measure of relief to several thousand Afghan Army troops and Soviet advisers in Khost.

The reports quoted travelers from the Paktia city of Gardez at the other end of the mountain road from Khost as saying that some relief columns had reached the outskirts of the besieged garrison town.

The battle for Khost, which reportedly is still under way, had been touted by Kabul and Soviet spokesmen as a test of the ability of Kabul forces to break the stranglehold of the Afghan guerrillas, or mujaheddin, on the city, which has been threatened since the outset of fighting eight years ago. Khost sits only a few miles from major mujaheddin staging areas in the Miram Shah area of Pakistan's Northwest Frontier Province.

According to well-informed sources, the mujaheddin had set up a standard wintertime siege similar to previous years but had added antiaircraft capacity close to the Khost airstrip, stopping almost all plane traffic to the town.

While the diplomats confirmed earlier reports of a break in the siege, they raised doubts about the volume of materiel that has reached Khost and noted that it would take a significant sustained commitment to keep open the 80-mile roadway through the mountains.

The diplomats said that the battle had caused significant casualties on both sides and that hospitals in Kabul were crowded with wounded.

Soviet and Afghan government spokesmen have claimed for several days that convoys have brought thousands of tons of relief supplies to Khost and that the mujaheddin have suffered casualties in the hundreds. Mujaheddin spokesmen have denied both claims, but the Pakistani press today reported that a mujaheddin commander in the battle area had confirmed through a Peshawar-based press agency that some Soviet units had broken through.

Diplomats who closely monitor the Afghan mujaheddin said it is premature to judge the effect of the battle on the guerrilla forces. They noted that fighting was continuing and that thousands of trained guerrillas and untrained forces from refugee camps in the border region and local tribes had flocked into the Khost area. They said it did not appear that the main Soviet and Afghan units had broken through to the Khost plain or that major operations had been launched to try to clear the area of all mujaheddin resistance.

"If they have succeeded at all," said one diplomat, "at enormous cost, they have gotten one convoy through."

The area of the most intense fighting to date has been along a roadway that snakes between rugged mountains for 30 or 40 miles before breaking out into the Khost plain.

According to one well-informed diplomat, the origin of the conflict probably lay in an effort earlier last summer by the government in Kabul to win over a major tribe that controls the Sadankamvan Pass, where the road enters the mountains. The pass -- narrow and twisting -- is thought impossible to negotiate without the agreement of the tribesmen.

When Afghan Army units first tried to enter the pass, they apparently found the tribesmen uncooperative. The relief columns, reportedly with significant Soviet involvement, then went south of the mountains along an old village road, joining up with the mountain road on the other side of the pass. This flanking maneuver was the first breakthrough of the relief effort, according to the diplomat.

From a small cluster of huts called Shabakatl, according to this account, Soviet helicopter-borne commandos leapfrogged mujaheddin units along the roadway for several miles and then pushed back toward the main column, fighting from two directions. The combined units then fought to Mirajan, about two-thirds of the way through the pass and close to a major mujaheddin camp.

It remains unclear, however, how much of the unit was able to negotiate the remaining few miles of mountain road and 20 miles of contested roadway to Nadev Shah, at the base of the mountains, and the Khost garrison.

The diplomat said the outcome of the battle still remains "murky" and that reports of just a few trucks and armored vehicles making it through to Khost left open a major question of what had happened to the bulk of the attacking column.