Afghan resistance fighters blew up several Soviet-built MiG fighter planes at an airport in western Afghanistan, destroying what was described as "a significant portion" of the Afghan Air Force, according to reports here today by western diplomatic sources.

"It's important. This is the biggest destruction of planes since the fighting began," said a long-time diplomatic specialist in Afghan affairs.

In Washington, a State Department official called the attack a "major blow" to the Soviet occupiers of Afghanistan by the U.S.-backed rebels, United Press International reported.

According to the reports reaching here, which cannot be independently verified, about 20 planes -- most likely Korean War-vintage MiG 21s that the Soviet Union has supplied Afghanistan -- were blown up last Wednesday at Shindand Air Force base.

Shindand is Afghanistan's second largest Air Force base, located south of Herat, near the border with Iran, and off the road to Kandahar, the country's second largest city southeast of Herat. The base is a long distance from the Afghan capital of Kabul, where western diplomats are based.

According to the diplomatic sources, the planes were blown up on the ground in a way that suggests sabotage rather than a battle with resistance fighters.

The Afghan Air Force, considered the elite of that country's military services, had been moving planes to Shindand from Herat's civilian airport and from Kandahar because security was considered better there, diplomatic sources said.

Shindand was jointly operated as an air base by Soviet forces, who invaded Afghanistan in December 1979, and the Afghan Air Force.

Usually, however, the Soviet and Afghan units are separated on the base, with Soviet soldiers responsible for guarding their section and Afghan military its portion of the field.

Diplomats here speculated that Afghan resistance fighters, who control much of the countryside in a bitter war that started even before the Soviet invasion, gained access to the planes with the cooperation of the Afghan guards.

The Afghan Army has been depleted by desertions, and is now reported down to about 40,000 men, many of them conscripts.

The Air Force, however, was described as the "darling" of the Afghan military. Its officers and men were said to have received extensive political training, mainly to ensure that the pilots do not desert with their planes.

"This must be a grave psychological blow to them as well as a setback to the Afghan military's air capability," said one diplomat here.

It appears that the Soviets have been using the Afghan Air Force more frequently in recent months, especially since the pilots have proven their political reliability, diplomatic sources said.

The Air Force has about 150 combat aircraft and 7,000 officers and men. Its planes are a mixture of Soviet-made equipment, including MiG 21s and MiG 17s.