ISLAMABAD, PAKISTAN, JAN. 27 -- Afghan guerrilla forces used new long-range heavy mortars for the first time in an attack 10 days ago that leveled a Soviet special forces camp in the Kunar Valley of Afghanistan, according to guerrillas and western diplomatic sources.

Use of the 120-mm mortars represents another escalation in military conflict between the Afghan resistance forces, who generally use guerrilla tactics, and Soviet and Afghan army units, who operate from fixed camps and generally move in well protected convoys.

Just as the introduction of Stinger missiles more than a year ago was designed to blunt the Soviet advantage in the use of air power against the guerrilla units, U.S. military strategists said the longer range and more accurate mortars will give the guerrillas, or mujaheddin, the capability of delivering damaging blows to Soviet and Afghan garrisons while remaining at a distance from the superior firepower of the regular Army units.

Introduction of the weapons at the current stage in the conflict also is a continuation of the "fight and talk" strategy that both sides seem to have adopted this winter. Nighttime temperatures in the mountains plunge in the winter and many peaks are covered with snow. In previous years, combat has diminished in the winter, but reports from inside Afghanistan indicate a steady level of battle coinciding with the current flurry of diplomatic maneuvering.

U.N. special negotiator Diego Cordovez continued his round of shuttle diplomacy, holding talks in Islamabad today with top Pakistani Foreign Ministry officials. In Peshawar, Yunis Khalis, the head of the seven-party anti-Soviet alliance, announced that the mujaheddin leaders now would be willing to meet with the U.N. undersecretary if he met two conditions.

Reversing an absolute rejection of the U.N. role in the Afghan conflict, the alliance leaders said they would meet with Cordovez if he announced publicly that the mujaheddin were one side of the Afghan problem and that each decision that is not acceptable to the mujaheddin is not valid.

Only a week ago, Khalis said the alliance rejected both the Geneva process and Cordovez's efforts, adding that they would talk only with the Soviet Union. The announcement provoked a sharp retort from moderate alliance leaders, saying it was unauthorized.

"It is a clear shift of the mujaheddin toward a more compromising position," said one diplomat who follows the talks.

Cordovez, who gave a bleak assessment of his current round of talks when he returned to Islamabad from Kabul yesterday, was said to be somewhat more hopeful today.

Cordovez would appear to have little problem in stating that the mujaheddin are one part of the Afghan problem, unless the alliance leaders are demanding that he stop dealing with the government in Kabul. Their intention was unclear in today's statement. Whether he could meet their other demand, publicly giving them a veto over his negotiations, is more problematic.

The mortars that were used in the Jan. 17 attack are supplied under the unacknowledged but widely reported U.S. assistance program for the Afghan guerillas, which last year totaled more than $600 million. U.S. specialists were considering a Spanish or an Israeli heavy mortar because of their lighter weight, which makes them more adaptable to mountainous guerrilla warfare.

The Spanish and Israeli 120-mm mortars, both of which are lighter than the comparable U.S. 107-mm mortar, have a range of roughly 3.8 miles. The Spanish mortar, which is also lighter than the Israeli, is more adaptable to guerrilla warfare in Afghanistan.

It was not immediately known which model had been sent.

The new mortars are part of a package of "new technology" that is reported to include sophisticated mine-exploding devices that are expected to be used by the Afghan guerrillas to try to get through mine fields surrounding Afghan and Soviet garrisons.

The new weapons were approved by the administration late last summer and they were initially expected to be delivered to the Afghan resistance groups by late fall.

The delivery was delayed for several months because of summit politics, tests on the new equipment and bureaucratic entanglements, according to U.S. officials and private American groups supporting the Afghan guerrillas.

In the Jan. 17 attack, the mujaheddin bombardment reportedly caught the Soviet special forces camp at Chrag Sarai in the Kunar Valley unaware and all of the camp's tents and some other structures were leveled, according to diplomatic sources. Western diplomats said it was not clear how many of the Soviet forces might have been on patrol at the time but that the camp is believed to have a complement of about 200.

Initial reports on the attack said that about 80 rockets were fired at the camp, causing a number of Soviet and Afghan army casualties, but mujaheddin sources involved in planning the operation confirmed today that the new mortars were used. The attack reportedly was carried out jointly by units from Yunis Khalis' Hezb-i-Islami and Syed Ahmed Gailani's National Islamic Front of Afghanistan. Units from the same two parties also operated jointly, or at least in neighboring sectors, during the recent battles around Khost, close to the Pakistan border.

That Khalis, a Moslem fundamentalist and the elected leader of the seven-party guerrilla alliance, and Gailani, a more moderate traditionalist, are cooperating militarily also could have implications for the political future of the alliance.

A planned operation in the Chrag Sarai region several months ago under the leadership of a commander from another guerrilla group reportedly was botched when much needed supplies and support units arrived as much as three days late.

Washington Post staff writer David B. Ottaway contributed to this story from Washington.