The Soviet Union's announcement that is withdrawing some military units from Afghanistan comes amid reports of a buildup of Soviet troops there in the past three weeks, an opposition strike and bloody feuding within the ruling party.

Most reports from the Afghan capital of Kabul are sketchy, and many cannot be independently confirmed. But, from the points they have in common, it appears that the Soviet occupation forces are increasingly hard pressed to control the insurgency by Afghan Moslems and to hold together the rival factions of their ally, the ruling People's Democratic Party.

Despite the Soviet announcement on the eve of the Venice summit of Western leaders, the reports suggest that Moscow is in no position or mood to withdraw significant numbers of Soviet troops from Afghanistan.

According to a Western diplomat from Kabul who briefed reporters in New Delhi Friday, the Soviets may have been augmenting their forces in Afghanistan shortly before the withdrawal annoucement.

He said the number of Soviet AN12 and AN22 transport planes landing at Kabul airport -- usually four to five a night -- has tripled or quadrupled in the past three weeks, Washington Post special correspondent Carol Honsa reported from New Delhi.

This appeared to raise the possibility that the forces the Soviets say they are withdrawing actually are being replaced as part of a rotation of troops.

The diplomat in New Delhi said that "a lot of [the Soviet aircraft] are carrying troops in," but that he did know whether they were replacements or additonal revivals.

"I have a hunch that the Soviets are beefing up their forces there," the diplomat said. He said that to win a Military victory, the Soviets "are going to have to change their tactics, and that will take them a lot more people."

Asked about the prospects of a political settlement, a regional specialist in New Delhi said, "Obviously there can't be any move toward a political settlement unless the Soviets choose to negotiate or move in that direction. So far, there have been no indications that they're after anything other than a military solution."

The diplomats said they believed that the Soviets now have considerably more troops in Afghanistan than the latest Western estimates of 80,000 to 100,000. The alleged buildup followed reports of increased rebel activity around Kabul.

Complicating the Soviets' efforts to crush reistance to Marxist rule in Afghanistan, which borders some of Moscow's own Moslem republics, has been fierce feuding within Afghan government ranks.

There are unconfirmed reports that key officials were either killed or wounded in a shootout in Kabul last week.

A usually reliable Afghan source in New Delhi told Honsa that rivalry between the Khalq (Masses) and Parcham (Banner) wings of the ruling party exploded in a gunfight early last week and that First Deputy Prime Minister Assadullah Sarwari and a former defense minister, Maj. Gen. Abdul Qader, were badly wounded.

Radio Afghanistan reported Wednesday that Qader had flown to the Soviet Union for medical treatment. The radio reported yesterday that Sarwari also had left by air for medical treatment in the Soviet Union. No other details were disclosed in either case.

In addition there have been rumors that President Babrak Karmal and Defense Minister Mohammed Rafi may have been involved in the shootout, but diplomatic sources said there were no indications that Babrak was harmed.

Babrak, the leader of the Parcham wing, was installed in a Soviet coup Dec. 27 in which his predecessor, Hafizullah Amin, the chief of the Khlaq faction, was killed. At the same time, Soviet forces poured across the northern border to help fight the anticommunist Afghan rebels and stabilize the new government.

"From all we know, the Soviets are trying desperately to keep the lid on this intraparty fighting," a Western diplomat in New Delhi said. He said his embassy is hearing reports of eight to 10 assassinations a night to the feud between the Parcham and Khalq factions. But he said he had no evidence that the Soviets are grooming replacements for Babrak or other afghan leaders.

Adding to the Kabul government's problems were a strike by merchants in the capital and renewed anti-Soviet student demonstrations, according to reports from Kabu.

These protests coincide with unconfirmed reports of an increase in isolated assassinations of Soviet soldiers in the capital, abductions of young Afghans at night by Army press gangs, shooting incidents at military installations and a rash of unexplained poisonings of school children.

The result has been a rise in tension in Kabul and more open opposition to the government.

Los Angeles Times correspondent Tyler Marshall reported yesterday from the Afghan capital that the almost total general strike entered its second day without any decisive government action to end it.

Diplomatic sources in Kabul said the strike, which began Saturday was organized by a mixture of Moslem religious leaders and dissidents, Marshall reported.

It was the second general strike in the capital since the Soviet invasion. But the Soviets this time appeared to be taking a much softer line than they did in February, when the first strike triggered on uprising in Kabul that was violently suppressed.