Soviet jets, helicopter gunships and armored vehicles have "reduced to rubble" 50 to 60 Afghan villages during the past two weeks in an apparent change of tactics to retaliate for rebel ambushes, an informed diplomatic source said today.

The raids have caused "thousands and thousands" of casulaties and sent streams of refugees toward the Afghan capital, Kabul, according to the diplomat, an area specialist from a noncommunist country.

He said a decreased reliance on ground action against Afghan Moslem rebels signaled a shift in the nearly seven-month-old Soviet military attempt to pacify Afghanistan and ensure the survival of its Soviet-installed Marxist government.

"Fewer Soviet troops are engaged, but more air power is being used," the source said. "That's a change, and it is a change that is seen and resented." s

He said there have been no recent reports as there were in May and June of Soviet search-and-destroy missions aimed at expanding their area of control in Afghanistan.

The source said the rebels currently are attacking the roads more actively than they have in the past, while the Soviets have little control over many of the major cities besides Kabul. He called Kandahar, Afghanistan's second-largest city, "virtually Indian territory" except for the airport and the military headquarters, which the Soviets control. Herat, the country's third-largest city near the Iranian border, is "a mess," he said.

At the same time, the diplomat noted increased activity at Kabul Airport over the past five days, with the Soviets apparently engaged in a massive resupply effort.

He said lighter, more mobile armored vehicles that will be able to pursue rebels off the roads into the untracked hills and valleys of Afghanistan were seen being unloaded from giant Soviet transport planes. Some of the equipment was also seen being reloaded on smaller planes, presumably to be sent out to Soviet bases away from the Afghan capital.

The diplomat said the attacks on Afghan villages recently were "either retaliatory raids or done to intimidate Afghans or done because the villages are suspected of harboring insurgents."

Some of the raids took place in areas where large Soviet supply convoys were ambushed by rebels within the past 10 days.

In one of those ambushes, the source said, 27 Soviet trucks, including some large petroleum tankers, were destroyed while heading east on a road from Bamian toward the new Soviet military airport at Bagram. Another ambush was near Gardez, south of Kabul in eastern Afghanistan near the Pakistani border.

Other reports reaching here have described Soviet retaliatory raids on villages, but most have emphasized that the number of casualties was slight -- mainly because villages were warned by rebels that an attack was probable.

Now, however, it appears that the Soviets are staging raids for intimidation purposes in villages farther away from the rebel attacks than before, the diplomatic source said.

He suggested that the stepped-up Soviet raids might also be aimed at trying to turn the Afghan population against the rebels, cut their supply lines and keep them off balance.

While the source acknowledged that his casualty figures were hard to back up, he said his figure of 50 to 60 villages destroyed in the raids "is not exaggerated. It is an understatement." He said it is based on eyewitness accounts reaching Kabul.

Among the villages he listed as being destroyed in Soviet raids were two in the Paghman area a short drive from Kabul, Bekhoot and Darrah.

Other villages hit in the massive aerial and armored attacks in the province of Kabul itself, he said, include Aab Darrah, destroyed July 1 by ground troops, armored columns and helicopter gunships with 38 casualties, including 12 rebel fighters; Guldara, bombed July 3, Baghram, bombed last Wednesday; and Gorbut, where eight insurgents were killed.

As a result of these raids, the source said, streams of refugees are moving toward Kabul, but authorities are refusing to allow nonresidents to enter the city. Refugees, afraid to return to their home villages, are camped on roads outside Kabul.

He said the government has imposed "Soviet-like internal passport controls" and insists on special permits to enter Kabul. Afghan sources here have reported in the past week that all entrances to Afghanistan's capital are closed.

A sharp change in the pattern of Soviet air transport activity at Kabul Airport started Thursday, when the big IL76 transport planes began landing during the day as well as at night.

He said as many as a dozen giant transports landed each day. Among cargo they were seen to unload were three types of light and mobile armored tracked vehicles: the low-slung, hatched BMD, the BMP mechanized infantry combat vehicle and the BRDM armored scout car.

They appear to be replacing the heavier tanks, which military experts say are almost useless with the poor roads and rocky terrain of Afghanistan.