An undisclosed number of Soviet military advisers, including some officers, were killed and one was taken prisoner in fighting with South African forces on a week-long incursion into Angola last week, South African Defense Minister Magnus Malan said today.

The discovery of Soviet advisers provides "undeniable evidence indicating Soviet Russia's involvement in the terrorist onslaught against South-West Africa," Malan said in a statement released in Cape Town.

The Soviet advisers were encountered "at SWAPO headquarters," Malan said, referring to the South-West Africa People's Organization, which is fighting South African troops for control of the territory of Namibia (South-West Africa).

"During the recent operations some of the Russian officers working together with SWAPO came into contact with us . . . . In the skirmishes some of these officers, among others, were killed," Malan said.

Military authorities refused to release the names or ranks of those killed, saying they wanted to notify the Soviet government first, but they said the man captured was a sergeant-major. They would not say how many had been killed, but said the number was fewer than five.

Although Malan contended that the discovery of the Soviet forces indicated a greater Soviet involvement than previously thought, it has been no secret that SWAPO has been supplied with Soviet arms since it began its bush war in 1965 to end South African administration of the territory, which was declared illegal by the United Nations in 1966.

Senior administration officials in Washington said today that while they had no independent confirmation of South Africa's claim that it had killed and captured Soviet advisers, they had no reason to doubt it. About a thousand Soviet and East German advisers are believed to be in Angola, working with the Angolan Army and SWAPO, the officials said, adding that they would not be surprised to learn that these advisers are working at the combat level.

South Africa's disclosure apparently is intended to help justify its incursion into Angola, which it said was aimed at destroying SWAPO camps and antiaircraft facilities operated by Angolan government forces.

It also may be intended to dampen criticism of the mild U.S. reaction to the incursion. Last night the United States split with its major allies to veto a U.N. Security Council resolution that would have condemned South Africa for invading Angola.

The South African statement did not say where the Soviets were killed, but observers consider it likely that the encounter happened at Xangongo, in Cunene Province. The South Africans had a major clash with Angolan government forces there and later showed visiting journalists a house they said had been occupied, until just hours before their attack Aug. 24, by 27 Soviet advisers.

"Earlier it was generally believed and claimed that Soviet Russia was only indirectly involved, but now the opposite has been proved," Malan said, "namely, the direct involvement of military personnel on the lower levels of terrorist movements. This is a tendency that gives rise to question marks."

"The South African government trusts that the Western world will take serious note of this and that this evidence will lead to a more balanced approach by the Western powers," he said.

South Africa appears to be attempting to gain acceptance in Western capitals, where a peace settlement is being pursued in a joint effort led by the United States, for its view that SWAPO's Soviet connection makes it a "terrorist" organization that cannot be a partner in negotiations.

The Western effort to secure a settlement on Namibia, involving first a cease-fire and then U.N.-supervised elections for an independent government, has always been based on the premise that SWAPO is a valid interlocutor in the talks and has a legitimate position.

Earlier this year, at a U.N.-sponsored peace conference in Geneva, SWAPO President Sam Nujoma offered to sign a cease-fire with South Africa but Pretoria declined, calling a settlement at that time "premature."

An official spokesman said that some South African soldiers were still in Angola today, at Ngiva, the Cunene provincial capital, 35 miles from the Namibian border. All troops left Xangongo, 60 miles inside Angola, by 4 p.m. yesterday, he said.

[Angola's ambassador to Portugal, Adriano Sebastiao, said in Lisbon that Cuban troops in Angola could intervene if South African forces penetrate as far as Huila, a town in the province of the same name, about 190 miles from the Namibian border, Agence France-Presse reported.]

[Sebastiao, speaking at a press conference, said Cuban troops had not yet come into action against the South Africans. He accused South Africa of trying to seize four southern provinces of Angola -- Cunene, Huila, Mocamedes and Cuando-Cubango -- to establish a buffer zone controlled by a "puppet administration" run by the Union for the Total Independence of Angola, a guerrilla faction headed by Jonas Savimbi that lost the Angolan civil war but controls portions of the south.]

[Earlier this summer, UNITA leaders said that the troops stationed at Xangongo included a Cuban battalion, but there has been no indication from South Africa that it encountered Cuban forces last week.]