The Soviet Union has intensified its arms buildup in Afghanistan over the past three days, including the landing of 25 large helicopters designed to drop Afghan troops into remote areas where rebel tribesmen are fighting for control, State Department officials said yesterday.

These officials it is unlikely that the Afghan military has the capability to fly the sophisticated Soviet helicopters, indicating that the Soviets will have to fly Afghan troops to the fighting.

This would mean a vastly increased Soviet role in Afghanistan's internal battles just four days after the United States warned Moscow to stay out of the fighting there.

Up until now, the 1,000 Soviet advisers to the pro-Soviet government of President Nur Mohammed Taraki, which seized power in a bloody coup almost a year ago, have been careful to avoid playing a combat role.

But ferry Afghan troops directly to the fighting places the Soviets as close to combat as they were a year ago in Ethiopia, when they landed troops near the front in fixed-wing planes and stationed advisers just behind the battle lines but left the actual fighting to Ethiopian and Cuban troops.

Bringing Afghan troops directly pockets of resistance in helicopters, though, increases the risk that Soviets will be forced into a direct involvement in the fighting there, officials here believe.

Although rebel tribesmen are battling the government authorities in two broad areas of the country, analysts here do not believe the Taraki-government is in danger of falling.

More likely, observes here believe, is the replacement of Taraki, who is not considered a strong leader. The leading contender is Afghanistan's foreign minister, Hafizullah Main, who Kabul radio reported yesterday was also named to the post of prime minister with sweeping power to remodel the government.

The Soviets have reacted strongly to these internal rebellions - by pouring vast amounts of military hardware into Afghanistan and by accusing the United States, Pakistan, Iran and Egypt of aiding the Tribesmen.

On Friday State Department spokesman Hodding Carter III warned the Soviets to stay out of Afghanistan's internal battles and denied that the United States was aiding the Afghan insurgents. At that time Carter said the Soviets had substantially increased their aircraft of military supplies to Afghanistan.

Since then, State Department sources said yesterday, those shipments have increased even more and further plane loads of arms are expected in the next few days.

During the past two days, State Department officials said, the Soviets have landed 25 huge cargo planes in Afghanistan's capital city of Kabul. Besides the helicopters, these planes brought in light tanks and nontracked armored personnel carriers.

"The Russians are trying very hard to hang on to their new found client," Afghanistan, said a State Department official.

Because the tribesmen are clustered in rough, hilly terrain that is made for ambushes, analysts here belive the troop-carrying helicopters will form the basis of the government's response.

With them, detachments of Afghan troops can be brough in quickly to attack rebel bases-a tactic that the United States tried without much success in Vietnam.

The pockets of rebellion are centered on the Western reaches of 4Afghanistan, around it third largest city, Herat, and in the east, near the Pakistan border not far from Kabul. The capital city is quiet.

Not much is known here about the status of the fighting. Government forces now claim to have control of Herat, which at one point last week was reported to be in rebel hands. Five American medical personnel in an eye hospital in Herat were reported safe by the State Department.

Afghanistan has shelled rebel training bases just over the border in Pakistan, near the city of Peshawar, where two or three rivals groups have set up headquarters to launch forays into Afghan territory through the Khyber Pass.

While Afghanistan has accused Pakistan of aiding the rebels, State Department officials believe all it has done is provide a safe haven for them.