The Soviets appear to be increasing their effort to turn over to Afghan troops more of the responsibility for fighting the rebels in Afghanistan, according to diplomatic reports reaching here.

Western military sources are doubtful, however, that the weak Afghan Army will be able to take on the additional burden.

The effort is preceived as the latest in a series of Soviet attempts to implement a policy they had hoped to carry out in the months just after they invaded Afghanistan in December 1979.

But the Afghan Army, weakened by purges, desertions and casualties to about one-third of the 90,000 soldiers it had, has proved imcapable of combating the stubborn guerrilla resistance that grips much of Afghanistan.

Most estimates put Soviet strength in Afghanistan at 85,000 troops.

Now, with cold weather slowing guerrilla activities, the Soviets reportedly have ordered Afghan units to take over more of the fighting.

Western diplomats say that the Soviet-supported government of Babrak Karmal approved on Jan. 18 a plan to use only Afghan soldiers against the guerrilla forces. The plan was not formally announced; officially, Soviet troops are not engaged in combat.

Western military experts believe Afghanistan's Army is far too weak to subdue or even control the insurgency without Soviet help. An example of that weakness is contained in a Western diplomatic report that described a recent five-day government sweep in Wardak Province, southwest of Kabul, to locate draft evaders.

The report said government troops carrying lists of names, apparently compiled by Marxists who used to live in the area, concentrated their search in and around four settlements.

After the search, the report said, the unit commander gathered local elders and informed them that the Army would not leave until the missing young men were produced. Many of the wanted men are believed to have fled into the surrounding hills or across the mountain passes to Pakistan.

Not long ago there were rumors of a government decree extending by six months the normal two-year tour for draftees, and the rumors were blamed for a riot in Kabul last month.

A tough new military service law was ratified by the government earlier this month, according to another diplomatic report. It said that the law provided that volunteers would be paid $50 to $75 a month, nearly 10 times the pay of a conscript. The draft age was said to have been lowered from 22 to 20.