About 4,000 Soviet soldiers have started evacuating their biggest garrison in eastern Afghanistan in preparation for a total pullout starting Sunday, guerrillas and diplomats said Thursday.

People coming from Jalalabad, site of the Soviet garrison, are reporting that "the Russians are sending their heavy equipment to Kabul," said Kabir, a member of the military committee of the Yunis Khalis guerrilla faction.Asked in his Peshawar, Pakistan, headquarters whether an airlift was under way he replied, "Yes."

In Moscow, the Soviet military said that foreign sponsors have stepped up weapons shipments to Afghan guerrillas and sent in more military advisers, endangering Soviet troops as they prepare to leave.

The Soviets intervened in Afghanistan in December 1979 and have been helping the Marxist government fight U.S.-backed Moslem rebels. Under an agreement signed last month in Geneva, the estimated 115,000 Red Army soldiers are to begin withdrawing May 15.

A Western diplomat, speaking on condition of anonymity, said guerrilla sources reported that up to 60 Soviet trucks headed west Thursday toward Kabul, 110 miles away.

He said he had no word on whether any Red Army troops had left yet, but said guerrilla sources reported shooting down an Mi-24 Soviet helicopter gunship.

"It would indicate the Soviets have resorted to using close air support again during their withdrawal," the diplomat said.

It is a tactic the Soviet-Afghan air forces had all but abandoned since the guerrillas acquired U.S.-made Stinger anti-aircraft missiles in 1986.

The sources said the Soviet 66th brigade, estimated at about 4,000 men, will be relocated to the Kabul area when they leave Jalalabad, just 30 miles from the Pakistan border.

They reportedly will be replaced by the Afghan 11th division, indicating a troop strength in excess of 10,000 men.

Jalalabad is the second major garrison the Soviets have evacuated since they left Barikot, farther north, on April 22.

Moscow's troops reportedly have handed dozens of posts over to Afghan troops in recent weeks. Well-armed guerrilla fighters have overrun many of them.