NEW DELHI, DEC. 29 -- Anti-Soviet mujaheddin are pouring fresh guerrillas into a major battle in Paktia and Logar provinces, setting up what is believed to be the biggest wintertime military confrontation in the eight-year Afghanistan war, according to reports from Pakistan and knowledgeable diplomats.

As Soviet-led forces attempt to punch their way through rugged mountains to relieve a besieged Afghan government garrison at Khost in Paktia Province, the mujaheddin are sending fresh units not only to the main battle site but also to adjacent Logar Province, through which Soviet and Afghan government forces and supplies must travel.

Reliable reports from Pakistan say that 3,000 mujaheddin are being sent to reinforce the 6,000 now operating around Khost and the 3,000 battling Soviet-led tank and elite troops along the Gardez-Khost highway. In addition, diplomatic reports say several thousand mujaheddin have moved into Logar Province, putting themselves into position both to maintain pressure on Kabul, the Afghan capital, and to harass traffic on the road to Khost.

Normally fighting in Afghanistan comes to a near halt by this time of year as cold and snow add to the difficulties of operating in the mountainous and barren Afghan landscape. With diplomatic efforts for a settlement of the Afghan conflict in an intense stage, however, both sides appear to be determined to press for a military advantage and to show no sign of weakness.

The Soviet-led operation, however, has puzzled U.S. government analysts, who predict that the attempt to break through will be very costly to Moscow and that the Soviet-Afghan forces will not be able to keep the roads open even if they do eventually lift the siege temporarily.

Whether by accident or design, the battle for Khost is taking place in one of the few areas where logistics favor the mujaheddin, rather than the mobile Soviet forces.

Not only is Khost close to the Pakistan border, but it also is only a few miles from major mujaheddin staging areas in northern Pakistan near Miram Shah and in the Parachinar area.

Soviet and Kabul government forces, on the other hand, are at the far end of a logistics chain running through miles of rugged mountain passes that offer the guerrilla forces both good cover and easy opportunities for mining the roadways.

Guerrilla forces have besieged Kabul's garrison in Khost almost from the beginning of the war, and it has been supplied either by heavily armed convoys or, more often, by air.

With the addition of highly effective Stinger missiles to the mujaheddin arsenal a year ago, the air route has become perilous and analysts raised the possibility this week that the latest Soviet-led offensive could be a desperation effort to get supplies into the area to avoid a severe defeat at this critical fight-and-talk stage of the war.

According to sources in Pakistan and informed diplomats, Soviet tank units operating in tandem with helicopter-borne special forces have been pushing their way along the 30-mile road from Gardez to Khost but have encountered heavy resistance from mujaheddin forces in the rugged passes that mark the route.

According to one report, the attacking units made good initial progress, pushing through the first of the passes beyond Gardez about a week and a half ago, but then were pushed back by the mujaheddin. As of Dec. 24, the attacking units still were stalled near their staging area around Gardez.

Since then, Kabul radio and the official Soviet news agency Tass have reported major breakthroughs along the Gardez-Khost highway and heavy losses among the mujaheddin. The mujaheddin commander in the area has labeled these reports as false, according to reports from Peshawar today.

{In the latest such claim, Soviet Foreign Ministry spokesman Gennadi Gerasimov declared the siege broken Tuesday, saying that "units fighting their way toward each other from the Afghan cities of Khost and Gardez linked up today," The Associated Press reported from Moscow.}

There have been no independent reports from journalists from either side, however, so it is impossible to verify the accounts. Kabul has allowed no western journalists access to fighting from its side. A half dozen journalists have been reported killed or captured in the past several months while accompanying guerrilla units.

Western diplomats and mujaheddin sources have acknowledged that the Soviet forces probably could push through the mountains into the broad Khost valley if they are willing to pour in enough manpower and take heavy losses. Once the Soviets cross the mountains, however, mujaheddin commanders reportedly have expressed confidence that they could close in behind, maintaining pressure on the units that have reached the garrison at Khost.

The Khost garrison is said to number several thousand government troops with Soviet advisers. The forces trying to break through the siege have been placed by western diplomats at about 20,000 Soviet troops as well as large numbers of government forces. Hospitals that normally handle Afghan Army casualties have been inundated in recent days and the special Soviet hospital in Kabul has been on high alert, according to western diplomats.

The town of Khost is located in a fertile valley that almost reaches the Pakistan border. During a government-sponsored visit there by western correspondents last summer, officials pointed out the remains of an Afghan aircraft they said was shot down by a Stinger missile. Residents of the town, which was very sparsely populated, said there were constant missile and mortar attacks by mujaheddin forces and many of the residents of the valley had fled. Many of the valley's fields were not being cultivated.

In addition to the heavy fighting in Paktia Province, western diplomats also have reported continuing heavy rocketing of the Kabul area in recent days as well as ground attacks on small posts on the capital's outskirts.

These actions also are unusual for this time of year and tend to confirm reports in Pakistan several weeks ago that there would be no lull in the fighting this winter.

The fighting comes against a backdrop of diplomatic maneuvering following the Washington summit meeting between President Reagan and Soviet leader Mikhail Gorbachev.

While berating the Soviets for not setting a specific date for pulling their estimated 120,000 troops out of Afghanistan, U.S. officials also are taking steps indicating that they expect new developments in the near future. Undersecretary of State Michael Armacost and key National Security Council staffer Robert Oakley are scheduled to be in Islamabad, the Pakistani capital, next week for strategy talks on how to coordinate their actions in the next round of diplomatic activity.

Soviet and United Nations officials have spoken of a possible February resumption of indirect talks in Geneva between the Afghan and Pakistani governments. Once those talks begin, developments could come very quickly.

Mujaheddin leaders in Peshawar have been nervous about the pace of diplomatic activity, warning that they must be included in any settlement.

While maintaining a hard line on a Soviet pullout, the rebels have made significant changes in their public position.

Mujaheddin spokesman Yunis Khalis, in a speech marking the eighth anniversary of the Soviet invasion, gave assurances that ways could be found to assure the safe withdrawal of Soviet forces and that most Afghans who had been "forced" to cooperate with the Russians could continue to stay in the country after a settlement. He said that only about 5,000-6,000 members of the communist People's Democratic Party of Afghanistan would have to leave with the Soviet forces.

This is believed to be the first time the mujaheddin have spoken publicly of such assurances and of the numbers of Afghans they would not allow to remain in the country.

In addition, Khalis spoke for the first time of an interim government, a concept that figures prominently in thinking among U.S. and Pakistani officials but had not been mentioned favorably by the mujaheddin.