Reports reaching here from diplomats and Afghans indicate that increasing numbers of Soviet soldiers occupying Afghanistan are engaged in its black-market activity.

They are reported to be selling guns and ammunition that rebel forces then use against them. These reports say the Soviets were also a source of gasoline during fuel shortages in Afghanistan's capital of Kabul.

Furthermore, according to two separate diplomatic sources today, Soviet soldiers are believed to have contributed to a wave of lawlessness in Kabul this winter -- including robberies and the looting of shops.

Analysts of the Soviet military say its top commanders also are concerned over what is said to be an increasing use of hashish and other drugs among the more than 85,000 troops in Afghanistan. There was no reliable indication of the extent of the problem, however.

The relatively cheap hashish, according to some sources, has replaced scarce vodka as the Soviet soldiers' recreational drug of choice.

It is the quest for drugs and Western concern consumer goods, readily available in Afghan cities though scarce in the Soviet Union, that feeds the military black market and more violent forms of lawlessness, according to observers here and in Washington.

Afghanistan long has been a source of drugs for the western world, and until the Marxist takeover in 1978 was a major stop along the trail of drug users. Now, together with Pakistan and Iran, it makes up the turbulent triangle that drug authorities say has replaced Southeast Asia as the major source of drugs ranging from hashish to heroin.

In an indication that drugs are easily available there, an Afghan rebel passing through New Delhi told correspondents in October that his band frequently acquires weapons by getting Soviet soldiers high on hashish and then taking their guns.

"It may sound exaggerated," he said, "but with one piece of hash we have been able to take so many weapons from the Russians."

The first of these reports seemed to be exaggerations. But, more recently, they are said to have formed a pattern indicating a breakdown in discipline of the Red Army.

In December, for instance, a diplomat reported hearing from a good source that a Soviet soldier sold a Kalashnikov automatic rifle for less than $1 worth of hashish. Asked if he would not get into trouble for selling his rifle, the Soviet replied that it was not his but he had stolen it from an Afghan storehouse.

Parts of Soviet uniforms, including winter overcoats and the red star that soldiers wear on their fur caps are reported to be rapidly available in Kabul markets.

Afghans, meanwhile, allege increased numbers of rapes by Soviet soldiers. In one incident cited, the disappearance of two Soviet soldiers was linked to the discovery in a field of two raped and slain Afghan school-girls.

"There is a real trend during the past few months of the discipline of the Soviet Army troops breaking down," said a close Western observer of the Red Army.

During the past two months stories in Kabul of looting by Soviet troops have multiplied. Late last year, diplomats in Kabul reported that two marauding Red Army soldiers were shot on the spot by their officers after forcing their way into the home of an Aghan government official. The soldiers, variously reported as being bent on rape or robbery, were said by Kabul sources to have murdered the government official, his wife, mother and three daughters when they resisted.

The Soviet soldiers were believed responsible for a wave of robberies in the Karte Parwan section of Kabul.

A diplomatic source said today that Soviet soldiers on patrol are responsible for the looting of houses and stores in Kabul. Another diplomat blamed Soviet soldiers along with "party thugs, armed freedom fighters and common criminals" for what he described as a spate of night robberies.

"The law and order situation in Kabul is deteriorating," concluded one of the diplomatic sources. He said residents rarely go out after 7 p.m. even though the curfew does not take effect until 11, "because in virtually every area of town break-ins after dark have become commonplace."

Only part of this is attributed to the Soviet troops, but they are now the principal authority in Afghanistan and have been known until now for their tight discipline.