Moslem fundamentalists have stepped up attacks on the Afghan capital and are poised for a major assault to bring down the communist government, sources said Friday.

More moderate guerrilla leaders predicted the attack would accomplish nothing except massive bloodshed and have begun to map out plans to stop them."It will be a massacre," said one rebel leader, who spoke on condition of anonymity. He said he feared his outspokenness would be punished with reduced arms shipments to his fighters.

There had been a period of relative calm in Kabul, the Afghan capital, until the fundamentalist guerrillas stepped up attacks. The government's well-equipped army and air force have responded with air and artillery strikes, diplomatic and guerrilla sources said.

At least 60 people have been killed and scores more injured in two weeks of heavy fighting.

There were reports that government troops had decimated two guerrilla columns marching toward the capital. Those reports could not be independently confirmed.

"We're attacking all the time but the time has not been set yet for the big offensive," said Hamid, a guerrilla spokesman who, like many Afghans, uses only one name.

According to the sources, Gulbuddin Hekmatyar, the most radical and anti-Western of U.S.-backed guerrilla leaders, has been chosen to lead the major assault on the capital of 1.5 million.

For more than 12 years, Pakistan's powerful intelligence agency has been a conduit for U.S. military aid to the guerrillas, who are fighting to unseat successive communist governments in their neighboring home-land.

Rumors have been circulating in this dusty border city for weeks that Hekmatyar and Shah Nawaz Tanai, the former defense minister of Afghanistan who defected to the guerrillas in March, were planning another coup attempt.

Tanai orchestrated a failed coup last March that left hundreds dead, destroyed sections of the capital and severely handicapped Afghanistan's air force before forces loyal to President Najibullah crushed the mutineers.