Soviet troops have started mining the mountain passes between Afghanistan and Pakistan, administration officials disclosed yesterday.

The Soviets obviously hope land mines will slow the traffic of Islamic guerrillas and arms between the two countries.

The planting of mines started about a week ago, in Kunar Province in northeastern Afghanistan, according to intelligence reports. The area has long been a stronghold of Islamic guerrillas.

U.S. analysts believe the Kunar operation will be expanded to other border provinces in an attempt by the Soviets to seal off Afghanistan's borders.

Up to now, it has been the Afghan guerrillas who have done most of the mining mostly of roads used by Soviet units.

It is believed Soviet troops dug in mines and warning devices in the Kunar passes, although some could have been sown by aircraft.

From a military point of view, Kunar is a difficult place for Soviet conventional forces to get at the guerrillas. Mountains and greenery provide good cover for the guerrillas and hard going for Soviet armor.

The United States resorted to mining during the Vietnam war in a vain effort to stop North Vietnamese troops and weaponry from moving south.

Kunar is the province nearest to Peshawar, Pakistan, where major guerrilla parties headquarter. Islamic guerrillas move between the two areas.

Kunar tribesmen gave the previous communist ruler in Kabul, President Hafizullah Amin, so much trouble that they are credited with impelling Moscow to replace him.

Pathan tribesmen in the hills and valleys of Kunar routed Amin's Afghan army and chased the communist bureaucrats out.

Last December, the Soviet army invaded Afghanistan, took control of Kabul, killed Amin and replaced him with Babrak Karmal.

But Kunar guerrillas kept fighting the Afghan army, with its Russian commanders, through the winter, bottling up units in mountains. The guerrillas reportedly had planned to attack in the spring.

But the Soviets did not give the guerrillas that chance. They attacked in force in the spring, hunting down by helicopter gunships those tribesmen they could not reach by road.

Since then, according to eyewitness reports, the Soviets have kept on the offensive in Kunar, bombing and burning villages where guerillas hide between forays.

Then new Soviet mines and warning devices in the passes from Kunar to Pakistan seem to indicate an attempt to consolidate the Russian hold on the troublesome province.

Some U.S. military analysts are predicting that the Soviets will step up their campaign in other Afghan provinces after the Summer Olympics, which begin later this month in Moscow.

According to the Pentagon, the Soviets have withdrawn fewer than 5,000 troops -- most of them antiaircraft and Frog missile units that were of little use in Afghanistan -- plus a regiment of tanks.

The current U.S. government estimate is that there are 81,000 Soviet troops in Afghanistan and 37,000 just over the Afghan-Soviet border.