The Soviet Union is "hurling" some of its "least capable divisions" into Afghanistan "and they are running into a substantial amount of trouble," Defense Secretary Caspar W. Weinberger said yesterday at a Pentagon news conference.

"It is widely known that you take the heaviest casualties in the beginning of any kind of conflict," Weinberger replied when asked why the Soviets would not be sending in their best troops.

He indicated that the Soviets are saving crack divisions for fights closer to home.

The Defense Intelligence Agency, in a book entitled "Soviet Military Power" and released yesterday, said the Soviets have 105,000 troops in Afghanistan, 25,000 to 30,000 more than were in the invasion force in 1979.

Despite this increase, made up mostly of security troops used to free regulars for combat duty, the DIA said, "The Soviets find themselves embroiled in a counterinsurgency campaign that cannot be won with current force levels."

The agency said that since December, 1979, 5,000 Soviet troops have been killed and about 10,000 wounded.

The young Soviet soldier sent to Afghanistan quickly become demoralized, the book said. "He is told by his superiors that he will be fighting against Chinese and Americans backing the Afghan counterrevolution," the DIA said, only to find that he is facing Afghans defending their homeland. Soviet troops sell arms and gasoline for alcohol, drugs and Western consumer goods, the agency reported.

"Poor morale also is evident on the battlefield where the soldiers hesitate to leave the relative safety of armored personnel carriers to close with a highly skilled and motivated foe, where the night belongs to the freedom fighters and where emphasis is placed upon indiscriminate use of firepower instead of sound infantry and combined arms tactics," the book said.

Despite such drawbacks, the DIA continued, the Soviets are learning how to fight counterinsurgency wars and building a pool of battle-tested officers, sergeants and infantrymen through the Afghan invasion.

The DIA warned that the Soviets eventually will use Afghanistan "as a potential staging area for power projection to Southwest Asia and to intimidate the regional states."