Uncertainty over the political leadership of Afghanistan deepened today as President Babrak Karmal failed to give a traditional address at anniversary celebrations of the communist takeover of the country eight years ago.

Kabul radio, monitored here, mentioned Babrak's name only once during a live broadcast of the capital's military parade celebrating the anniversary of the Afghan communist coup d'etat. The radio failed to state whether Babrak was at the parade, an omission that sparked new questions about his whereabouts since his reported departure for Moscow on March 30.

Western diplomats here have suggested that Babrak, 57, may be receiving medical treatment in the Soviet Union for an unspecified lung ailment. But an article in today's editions of the official Soviet Communist Party newspaper Pravda appeared to criticize Babrak's leadership, intensifying speculation that the Soviets are displeased with Babrak and may remove him.

The latest signals of Soviet displeasure come before the scheduled opening May 5 of a new round of indirect talks under U.N. auspices between Pakistan and Afghanistan to focus on terms for a Soviet troop withdrawal from Afghanistan and an end to Pakistani and U.S. support for Afghan rebels. It was not clear what effect the uncertainty over Babrak's whereabouts would have on the talks. If they are successful, a key question would be the nature of the government in Kabul once Soviet forces leave.

During the past year, diplomats often have questioned whether Babrak might be falling out of favor in Moscow. Those observers note that the Soviet-dominated Afghan government has shifted its strategy for generating popular support and that factions within the ruling communist party have continued a longstanding feud that involves assassinations of rival officials.

["There is no question that the Soviets are unhappy," a State Department specialists on the region said Sunday in Washington, "and they may be looking for alternative leadership to be more effective in Kabul."]

In an article commemorating the coup, Pravda said, "Dissatisfaction with what has been done and sharp criticism of failings that have hindered the revolutionary process in Afghanistan could be seen in the decisions taken last autumn by [the Afghan communist party's] Revolutionary Council . . . and in the theses on the necessity to widen the social basis of the revolution."

The article was taken as the latest in a series of recent signals from the Kremlin that it is not happy with Babrak. In February, Soviet leader Mikhail Gorbachev snubbed Babrak by failing to meet him during the Soviet Communist Party Congress in Moscow.

Last week, Afghan Prime Minister Soltan Ali Keshtmand, formally the country's number two leader, received an unusually warm reception in Moscow. Keshtmand and the former Afghan secret police chief, Najibullah, are generally regarded as the most likely successors to Babrak.

Both Keshtmand and Najibullah have received unusually close attention from the official Afghan media and have replaced Babrak at various official functions since he disappeared from public view last month. At today's military parade, neither man was chosen to replace Babrak as the keynote speaker -- a task performed by Defense Minister Lt. Gen. Nazar Mohammad, who is not seen as a potential successor.

The Soviet Union installed Babrak as president of Afghanistan shortly after it invaded the country in December 1979, to replace the collapsing communist government of Hafizullah Amin. Babrak remains in power with the support of about 115,000 Soviet troops who lead the fight against a broad Moslem resistance movement.

Babrak heads the Parcham faction of the ruling People's Democratic Party of Afghanistan, installed by the Soviets after the more hard-line Khalq faction of Amin had bitterly alienated the overwhelmingly traditional and rural population with forced, radical social changes.

Since the invasion, the Soviet and Afghan troops have managed to impose military control over the major towns. But Babrak, in his six years in office, has failed to extend the party's base to any major segments of the rural population.

In the past year, the party has made strenuous new attempts to recruit rural support through tribal assemblies and appeals to nonparty members. In an interview in March with the Moscow weekly New Times, Babrak said the top councils of both the party and government had been substantially enlarged.

"There has been no visible evidence that all of this maneuvering has gotten the party anywhere," a western analyst said here last week.

Since Kabul radio reported Babrak's departure for a "short, unofficial visit" to Moscow on March 30, Babrak has not been seen in public, although the state media have reported his signature on ceremonial declarations and messages to foreign governments.

Despite the signs that Babrak may be losing Soviet support, he has not altogether disappeared in Kabul and Moscow. Students carried a large portrait of Babrak in yesterday's parade in Kabul, the only point at which radio announcers mentioned his name.

Western diplomats who have suggested that Babrak's absence may be for medical reasons noted that Babrak's health has been questioned before and that he spent nearly three weeks in the Soviet Union last summer for what officially was called a general medical examination. In an interview last month with a British journalist, Babrak laughed off rumors that his health was failing.