A series of Afghan rebel victories and a steady barrage of rocket attacks on the Afghan capital have led Soviet troops to take the lead in defending Kabul, Western diplomats say.

The Soviet move raises new doubts that President Najib's Marxist government can survive after Moscow withdraws its more than 100,000 troops from Afghanistan, the diplomats said Thursday.The rocket strikes this week suggest rebel supplies have improved and their commanders around the city are better coordinated, said the diplomats, who spoke on condition of anonymity.

They quoted one unconfirmed report as saying 4,000 Afghan guerrillas have massed on the western edge of the capital and said fighting was reported on all fronts around Kabul.

Soviet forces already provide most of Kabul's air and artillery defense, but Soviet troops now are moving to the capital's perimeter to battle rebels, the diplomats said.

They said Soviet troops were sent to Kargha, three miles west of Kabul, after Afghan troops suffered setbacks. They gave no details.

The Soviet and Afghan governments maintain that the withdrawing Soviets are letting Afghan troops take the lead in fighting the civil war.

Red Army troops entered Afghanistan in December 1979 to help the government fight U.S.-backed Moslem guerrillas. The Soviet Union agreed in April to withdraw its troops, and the withdrawal began in May.

Rockets blasted the capital Tuesday and Wednesday.

Radio Kabul quoted Afghanistan's official Bakhtar news agency as saying shells hit residential areas of the country's eastern city of Jalalabad Thursday. The broadcast said five people were killed.

Official Soviet and Afghan media blame the rocket attacks on Afghan guerrillas.

Abdul Haq, believed to be the strongest guerrilla commander in the capital area, said residential areas are not targeted, but acknowledged some rockets go astray.