The Soviet Union sent more military advisers today to Lubango, the nerve center for the Angolan Army's increasingly serious war against South Africa in the southern part of this country.

The arrival of three Soviet advisers, adding to the approximately 20 known to be here, plus the confirmed presence of some Cuban troops close to the war front, appeared to increase the possibility that the volatile southern African region could become a center of East-West conflict.

The Soviets, along with about 200 Angolan troops, flew here just one day after South Africa announced that it had captured a Soviet adviser and killed several others in fighting against what it called a combined force of Angolans and Namibian guerrillas last week in southern Angola. Angola denies that there are any guerrillas within 200 miles of the border of Namibia.

South Africa has defied U.N. directives that it end its administration of Namibia (Southwest Africa) and it has charged that guerrillas of the Southwest Africa People's Organization, who are fighting for independence, are operating out of bases in southern Angola.

The 20 Soviet advisers can be seen each day in Lubango, about 100 miles north of the Namibian border, as they take their meals at the once-fashionable Grande Hotel da Huila. They stay close together and wear sidearms and the same Eastern Bloc camouflage uniforms as the Angolans but without markings of rank.

Angola acknowledges having Soviet advisers and says they are necessary because the country is under increasingly heavy military attack from South Africa.

The United States estimates that there are about 15,000 Cuban troops and fewer than 1,000 Soviet advisers in Angola. Cuban troops have been stationed here for about six years. The presence of communist forces has become a barrier to normalization of relations with the United States, which Angola seeks. Angola has said the Cubans will leave once independence is achieved for Namibia, which borders it on the south, but that if South Africans continue to invade, it will ask for assistance.

The Angolan Cabinet met in Luanda today and the Angolan news agency Angop said later that the country felt "conditions" were right for invoking mutual defense treaties with the Soviet Union and Cuba as well as Article 51 of the U.N. Charter, which provides for collective defense of a member facing outside aggression. But there was no indication that Angola was actually invoking the treaties.

As the only Western journalist allowed into the war area since the start of the current fighting last week, I found my movements severely restricted by Angolan authorities. The provincial director of information for Angola's ruling party, Joao Castro, ordered me today to stop reporting and put me on the first plane back to Luanda, the capital. Lubango is the capital of Huila Province, one of four in the south placed under a state of emergency.

I saw the three arriving advisers, carrying briefcases and AK47 rifles with polished wooden stocks, and the Angolan troops while I was waiting at the Lubango airport for a flight to Luanda. All flights now are military, as commercial service has been stopped for almost two weeks because, according to Angola, South Africa is interfering with airspace.

Diplomats concurred that the three advisers were Soviets. They were separated from the Angolan troops when they left the plane and were greeted by a Soviet adviser based in Lubango.

In Lubango, Angolans were somewhat miffed by South Africa's trumpeting of the capture of a Soviet adviser and the killing of others during the invasion. They heard the news by international means. The Angolan radio has yet to report on the South African claim.

South Africa contends that the Soviet presence is proof of Soviet collusion with SWAPO, but most African nations, even the moderate ones, line up on Angola's side, saying that South Africa is the aggressor because of its frequent attacks into Angola.

The African nations maintain that SWAPO has the right to carry out attacks into Namibia from Angolan soil because the South African occupation is illegal and the United Nations has recognized SWAPO as the sole legitimate authority in Namibia.

The U.S. decision to stand alone Monday and veto a Security Council condemnation of the South African invasion is expected to cause considerable African criticism and decrease any possibility for improvement in relations with Angola. It could also lead to an increased Soviet-Cuban presence, although Western diplomats say they believe Angola is trying to avoid such a move.

Castro, the information director here, said: "Angola is an independent country. We are free to choose our friends. They are helping us."

He was speaking at dinner in the Grande Hotel where the Soviets were having their evening meal.

The apparent commander of the Soviet group at the hotel is over six feet tall and about 200 pounds, with a large shock of wavy, graying hair. He always sits at the head of a table of about 10 Soviets, occasionally joined by one Angolan. The other Soviets sit in groups of two, four or six without mixing with Angolans.

It appears that few of the Soviets speak Portuguese. They also refused to speak to me.

A number of the advisers seem to be well into middle age, with thinning hair and paunches. Some carry briefcases and if they were not wearing camouflage uniforms many could pass as the typical American traveling salesman.

The Soviets stand out because of their inability to speak Portuguese, their size and their light complexion. There are whites in the Angolan Army because of the Portuguese colonial past, but most are darker and smaller than the Soviets.

Soviet civilian advisers in Lubango make no effort to keep a low profile. Two of them, Alexander Bronchatov, an official of Komsomol, the Soviet Communist Party youth organization, and Sergei Krylov, a physician, were in the reception line at the ornate mansion of the former Portuguese governor to welcome a U.S. congressional delegation led by Rep. Howard Wolpe (D-Mich.) last month.

Bronchatov, who speaks vernacular American English although he said he has never been to the United States, told of his hopes for working with the youth wing of the country's ruling party, the Popular Movement for the Liberation of Angola.

He does not speak Portuguese. "It's quite a problem," he said.

Speaking before the recent South African incursion, Krylov, the physician, said that the hospital in Lubango handles "quite a lot of war casualties" including victims of an antigovernment guerrilla group supported by South Africa.

The Grande Hotel where the Soviet military advisers are staying was a favorite during colonial days of South African tourists. South African men often found it a convenient place to escape Pretoria's rigid laws against interracial sexual relations.

Today the swimming pool is empty, the bar is usually closed and running water is a rarity.