Afghan President Babrak Karmal undertakes a mission to Moscow next week that diplomats here and in Kabul believe is designed to give his embattled regime an aura of legitimacy with the approach of a series of international meetings expected to discuss the Soviet role in Afghanistan.

His trip comes at a time when rebel activity in Kabul reportedly is intensifying and the Afghan Army is said to have deteriorated to the point of being able to carry only a minimal share of the fighting.

Moreover, according to recent reports, ambushes are now occurring regularly east of Kabul on the important highway from the capital through Jalalabad and the Khyber Pass into Pakistan. The stretch on the western end of the highway had been considered safe until recently.

Nonetheless, according to diplomatic sources here and in Kabul, the Soviets are attempting to give the impression that all is well in Afghanistan by inviting Babrak to Moscow at this time.

"I assume that what they are trying to do is give him a little bit more exposure and legitimacy. I think they are trying to get it across that he is the guy in charge, the leader of the nation," said one diplomat.

"Having Babrak come and say everything is wonderful and normal gives the Soviets another handle to say that these [upcoming] meetings need not discuss the subject."

The Soviets have embarked on a major diplomatic campaign to get Afghanistan off the agenda of diplomatic conferences and, if they do not succeed in that, at least to have the debate muted.

The Soviets are faced, however, with a full-scale U.N. General Assembly debate on Afghanistan next month. Presently, preliminary meetings to fix the agenda of the Madrid follow-up conference on the Helsinki human rights accords are stalled over, among other issues, the implications of the Soviet invasion of Afghanistan last December.

A nonaligned conference is scheduled here early next year at which some nations are sure to bring up the fate of Afghanistan -- which was considered a nonaligned country.

The Soviet Union could more easily mute the debate on Afghanistan at those meetings, according to analysts here, if they could fix an image of Babrak as the leader of an independent state with the Soviet troops there merely to protect against outside interference.

Apparently in search of that image, Moscow and Kabul have intensified their attacks on the United States, China and Pakistan as aggressors in Afghanistan.

"They are keeping that up," said a diplomat here, "because that is the only basis for their being there."

Traditionally, the Soviets have used international meetings to promote their objectives, but they have run into growing opposition over the invasion of Afghanistan. Fellow communists from Cuba and Romania voted in favor of a resolution at an interparliamentary meeting in East Berlin last month that called for Soviet withdrawal from Afghanistan.

At a Commonwealth Parliamentary Union meeting in Lusaka, Zambia, this week, India was the only country to give any support to the Soviet position, according to India's former foreign minister, A. B. Vajpayee. He said in an account published this morning in the Indian Express that his delegation was "taunted" by other nations, who said Indira Gandhi's government was "underwriting the Soviets' armed intervention" by not condemning it.

Soviet President Leonid Brezhnev is scheduled to visit New Delhi late next month, reportedly to stiffen India's resolve to keep the nonaligned conference from taking too strong a stand on Afghanistan.

While India has said the Soviet troops should be withdrawn, the Gandhi government has insisted that Moscow's move into Afghanistan is part of the big-power rivalry in the area, implying that the Soviets were reacting to U.S. activities.

The last Afghan president to visit Moscow, Nur Mohammed Taraki, was assassinated shortly after his return to Kabul by Hafizullah Amin, his partner in the April 1978 coup that placed a communist government in power, Amin was killed in the Soviet-assisted takeover that put Babrak in power.

Most experts here discount rumors circulating in Kabul and among Afghan emigres here that the Moscow trip may also prove to be Babrak's undoing. However, diplomatic and journalistic reports from Kabul do indicate that rebel attacks in and around the capital are increasing in number and in boldness.

Kuldip Nayar, an Indian journalist who has just returned from six days in Kabul, wrote this week that there are 20,000 rebels in the city of 1 million, and that they have informers within the police and the Army.

He said the Indian ambassador in Kabul and his wife were caught in a crossfire between soldiers and rebels while going to their home in the city center one night and a friend's auto was shot at in the outskirts by rebels who thought it must be a high government official's car.