The sounds of jetfighters screeching overhead and the resounding thuds of bombs crashing to the ground echo continually throughout this vast rebel-held garrison.

The guerrillas dive for cover, but each time the blasts claim more casualties from among the 500 or so fighters occupying the base.Government forces are also pounding the camp with rockets fired from Jalalabad, 8 miles away, and occasionally with powerful Scud missiles from Kabul.

The rebels respond with artillery. Sophisticated U.S.-supplied Stinger missiles down one or two of the scores of Soviet-made MiG jetfighters that daily attack the rebel positions around Jalalabad.

At the airport, about 2 miles away, troops and guerrillas face each other a few hundred yards apart on either side of a heavily mined no-man's land. Rebels in the Pakistan frontier city of Peshawar said heavy fighting at the facility Saturday left 20 guerrillas dead, and that the government has sent reinforcements to the airport.

Almost two weeks after launching a massive offensive against the strategic eastern city of Jalalabad, the U.S.-backed Islamic rebels known as Mujahideen or "holy fighters" are bogged down and suffering heavy casualties despite initial successes.

Hospital and rebel sources in Peshawar said Saturday more than 150 guerrillas have died and 564 have been wounded since the operation began March 6.

"The Mujahideen miscalculated the strength of the regime in Jalalabad," Commander Anwari of the Iran-based faction of the Afghan rebel alliance said in Peshawar.

"We are fighting a well-entrenched enemy," said Lal Mohammad, a 35-year-old rebel commander at Samarkhel. "We are suffering heavy casualties here at the rear from bombs and artillery."

But, he added, "We were prepared for this. We can continue even for another two months."

Jalalabad would be the first major city to fall to the guerrillas in the 10-year-old civil war. Its capture is seen as crucial to rebel hopes of eventually taking Kabul, 75 miles to the west. Diplomats had predicted a rapid fall for the communist government of President Najibullah following the Feb. 15 withdrawal of Soviet troops.

The Pakistan-based resistance, which also hopes to establish its alternative government in Jalalabad, believes the city's fall would prompt wide-scale defections among frightened government troops.

But although rebel morale remains high and government casualties have also reportedly been heavy, a prolonged stalemate in Jalalabad would boost government hopes and lead to frustration among the already fractious guerrilla groups.

The Samarkhel garrison, which was intended to protect the city, was taken in just three hours on March 10. It is now the main rebel base for the operation against Jalalabad.

The rebels captured about 40 tanks as well as other military vehicles and weapons when they seized the garrison, located on a five-acre site shaded by palm and pine trees and surrounded by barbed wire.

Paths and open ground throughout the camp are covered with bomb craters, one of which contained body parts sticking up from beneath the earth.

The scene in the former officers' family quarters bears witness to the hasty government retreat and the undisciplined rage of the Mujahideen.

In every room, trunks, suitcases and cupboards were broken into. Piles of clothes, bedding, smashed furniture, children's toys and baby cribs, letters, photographs and other personal belongings lie strewn on the floors with broken window glass and half-eaten meals crawling with flies. The stench of dead bodies pervades the building.

Outside, refrigerators and air conditioners that have been thrown from the building lie smashed on the ground.