U.S.-backed Afghan insurgents have inflicted the highest casualties ever on elite Soviet troops trying to cut off guerrilla supply routes along the Pakistani border during the past six weeks, according to a senior administration official.

The official described the fighting as the most intense since the rebels began opposing the communist government installed in a 1978 military coup. Soviet troops invaded Afghanistan in December 1979 to support the government.

The latest fighting comes against the backdrop of apparent determination by both the United States and the Soviet Union to intensify the military confrontation even as long-term maneuvering has begun for a possible political settlement to the war.

The official said that the administration is determined to continue increasing the amount and sophistication of American weapons supplied to the insurgents, known as mujaheddin, until it sees "serious indications" that the Soviets are ready to withdraw their troops -- estimated by U.S. officials at 115,000 to 120,000 -- from Afghanistan.

"What we're doing is matching," said the official, who took sharp issue with Soviet complaints about the recent supply of large numbers of U.S.-made Stinger antiaircraft missiles to the Afghan insurgents.

In fighting during the first two weeks of June, he said, the insurgents killed or wounded between 250 and 300 Soviet special forces, known as spetsnaz, and shot down 15 Soviet or Afghan aircraft in fighting around Ali Khayl in Paktia Province.

The Soviets lost 120 special forces between May 23 and June 5, the official said, in the same area where they tried in vain to occupy a guerrilla supply route and points leading from Peshawar into eastern Afghanistan.

The official, echoing U.S. intelligence reports, said the recent battles were between large guerrilla units from five of the seven factions making up the U.S.-backed Afghan Alliance and a full division of Soviet and Afghan government troops including a major artillery component.

The Soviet attacks on guerrilla resupply bases in Paktia Province followed intensive air raids on refugee camps and guerrilla staging areas inside Pakistan this spring aimed at stopping the insurgents' expected summer offensive before it could get under way.

Last summer, the Soviets used the same tactic with considerable success, occupying one major rebel camp and throwing the insurgents off balance until late summer.

This year, the senior U.S. official said, the guerrillas not only forced the Soviets and their Afghan allies to retreat in Paktia, but also succeeded in setting up alternative routes farther north to ensure the continued flow of arms and supplies into the interior of the country.

This year's intensified fighting follows a pattern of escalation that began with last year's Soviet offensive, which included the introduction of new Soviet tactics based on the use of more sophisticated weapons and highly mobile spetsnaz units.

The Soviet escalation was one important factor in the administration's decision to begin sending Stingers to the insurgents.

The rebels received a shipment of 150 Stingers late last summer after selected units were trained to use the shoulder-fired missiles. They were sent another shipment of 600 earlier this year.

To the surprise of U.S. military analysts, there is still no evidence the Soviets have devised countermeasures to protect their aircraft from the Stinger, whose sophisticated guidance system homes in on infrared or ultraviolet energy rather than the heat emitted by a plane's jet engines.

The failure is particularly surprising to these analysts, given the high rate of the Soviets' aircraft losses -- an average of one a day since October, according to U.S. officials.

The level of sophistication of the weapons being used by the two sides may now be on the verge of rising in a new cycle.

U.S. defense officials are examining an array of American and other western-made artillery, mortars and antitank weapons that they feel would be helpful to the guerrillas in the fighting, according to another administration source.

The only limit being placed on the kind of weapons the United States is now willing to send the seven guerrilla factions is their suitability to the terrain and conditions of warfare, this source said.

"We're going to give them whatever they need to do it," he said.

The administration is maintaining an equally firm line on the political side following signals beginning more than a year ago that the Soviets might be considering an end to their occupation of Afghanistan. The latest such signal is an attempt by the Soviets and their Afghan allies in Kabul to form a "government of national reconciliation" that would include some opposition elements.

The rebel Alliance and U.S. officials have been skeptical of the Soviet initiative and question whether Moscow has made the hard decisions that would lead to a settlement acceptable to the Afghan resistance groups and the more than 3 million refugees living in Pakistan and Iran.

"Like the North Vietnamese, the Afghans are very determined people with a will to fight," the senior official said. "There will have to be a recognition of the political realities of Afghanistan before there can be a settlement."

Asked what the administration would regard as a "serious indication" of the Soviets' sincerity, the official said, "when they begin to talk to the Pakistanis about a short timetable for a troop withdrawal . . . or they start talking about a government not dominated, or controlled, by the Kabul regime -- a government acceptable to the resistance and refugees."

Another U.S. official said that Soviet indications of a willingness to withdraw from Afghanistan have had the ironic effect of convincing the last skeptics within the administration that the U.S. policy of escalating aid to the mujaheddin is working.

Soviet officials have complained bitterly in recent months about the administration's decision to introduce the Stinger, particularly in large numbers, starting early this spring.

They say the U.S. pressure has come just when the Soviets have begun to consider seriously a withdrawal from Afghanistan and has made such a retreat more difficult.

One U.S. official said that now there is virtually no opposition from any of the U.S. intelligence agencies or departments involved to proposals for sending new kinds of weapons to the Afghan resistance movement.

In addition, he said, the success of the Afghan guerrillas in using the Stingers has helped to convince skeptics within the administration that the rebels may actually have a chance to succeed in wearing down the Soviets and persuading them to withdraw.