The rebel-held garrison at Samarkhel, the once-bustling headquarters for the U.S.-backed guerrilla offensive against the eastern city of Jalalabad, has been reduced to a ghost town.

Heavy air and artillery attacks by government forces have devastated the sprawling camp. In addition, hundreds of rebels have left to celebrate the Islamic festival of Eid with their families, prompting fears of a government counteroffensive.The guerrillas occupied Samarkhel on March 10, four days after the start of their assault on Jalalabad, 70 miles east of Kabul.

The offensive was the first attempt by the U.S.-backed guerrillas, known as Mujahideen, to stage a frontal assault against a major city in their 10-year-old war against the pro-Soviet government in Kabul. Although they took Samarkhel in just three hours, the battle for Jalalabad quickly turned into a bloody stalemate.

One rebel commander said Friday 120 guerrillas have died and between 250 and 300 have been injured at Samarkhel since it was captured. He said 17 were killed and scores injured recently when a bomb hit a huge ammunition dump in the garrison.

Rebel commanders conceded that many of the casualties were the result of recklessness by the guerrillas, who believe it is cowardly to take cover during air or artillery attacks.

"Everyone must die on an appointed day, so why should I worry," said one rebel fighter, Khatamul Shinwari.

The commanders said they have limited guerrilla activities around Jalalabad and elsewhere during the holy month of Ramadan, when Moslems are required to fast from sunrise to sunset.

But they said hundreds of guerrillas are now leaving the war front to be with their families in refugee camps in Pakistan during the three-day Eid festival, which began Saturday and marks the end of Ramadan.

The commanders, fearing the government may exploit the situation and stage a counteroffensive against the depleted rebel forces, are desperately seeking replacements.

"I'm going to the border villages and to refugee camps in Pakistan to find men to replace those at the front line," said Commander Rahm Dil. But he conceded it was difficult to prevent guerrillas from returning home for Eid, the most important Islamic festival of the year.

Western diplomats in Pakistan say the government's success in defending Jalalabad has boosted morale among members of the ruling party of President Najibullah in Kabul despite the Feb. 15 withdrawal of Soviet forces backing the regime.

But Mujahideen at Samarkhel, 8 miles southeast of Jalalabad, said their political leaders based in Pakistan raised false hopes of a quick victory.

"You must realize that the men who are fighting are a better judge of their military strength and we knew it would not be as quick and easy as our political leaders had made it out to be," said one young guerrilla, Nadir Khan, toting a captured Soviet Kalashnikov rifle on his shoulder.

Others said the city could have been taken several weeks ago if the rival rebel groups in the area had molded themselves into a disciplined fighting force.