The mining of main trails between Afghanistan and Pakistan by Soviet forces has been effective in slowing the traffic of guerrillas and supplies between those two countries, and expansion of the mining operation "will be a major hindrance" to that movement, U.S. officials said yesterday.

The Soviets, in an effort to shut off aid reaching rebels fighting the Soviet army, began dropping mines from aircraft last week, mostly in Kunar province in northeastern Afghanistan.

U.S. officials said they believe the Soviets have started mining in other sectors as well and that the tactic probably will be employed on all major trails.

This will not end the traffic, because there are hundreds of secondary mountain trails across the long, rugged border. But it will delay getting supplies through, because it takes longer to use these secondary trails.

In a briefing yesterday, U.S. officials said that for the past few months the situation has remained largely a stalemate, with the Soviets in control of the territory they occupy but unable to end the widespread rebel resistance or develop political support for the Soviet-installed government in Kabul.

Meanwhile, there are signs that the Soviets are trying to adapt to fighting a guerrilla war.

They have reorganized their forces in the country, splitting some divisions into four or five units of about 2,500 to 3,500 men each, which could be moved quickly to meet a large-scale rebel attack.

Soviet units also reportedly have orders allowing them to fire more freely into villages to drive out rebels, a tactic that may account for the steady rise in the number of Afghan refugees.

The number of Afghans who have fled in the past six months is "considerably over 1 million," officials said, with 850,000 of them in Pakistan.

The Soviets have markedly increased their fleet of helicopter gunships in Afghanistan, officials said, though there is no evidence of antiaircraft missiles being slipped to the rebel forces to help them combat the weapon they fear the most.

Officials said truck and air transport traffic into Afghanistan from the Soviet Union was especially heavy in June, with much of it at night, so that it is hard to say if the Soviets have reduced troop strength despite an announced withdrawal of 5,000 men.