In a military move that could undermine Western diplomatic efforts to avoid immediate confrontation with African countries at the Security Council, South Africa has begun a major build-up of troops in the northern part of Namibia, according to Western intelligence reports.

The South African move, which reportedly involves two new regiment-sized units, is being matched on the other side of the Angola-Namibia frontier by a general mobilization of Angola's army and militia and reinforcement of Cuban troops in that area, according to other diplomatic sources.

Involved Western diplomats expressed sharp concern about the impact the mutual escalation could have on their continuing efforts here to negotiate a compromise resolution to replace African demands for global economic sanctions against Pretoria in the dispute over Namibia, a mandated territory ruled by South Africa since 1920 and also known as Southwest Africa.

The negotiations continued yesterday as the African countries - who have shown on enthusiasm for putting the United States and its allies in the difficult position of having to choose between joining or vetoing the sanctions - once again backed away from a showdown.

They agreed to postponing a scheduled formal Security Council session and continued informal talks with the Western negotiating team composed of the United States, Britain, Canada, France and West Germany.

The military build-up in the Ovambo tribal area - split by the Namibia-Angola frontier - appears to be tied to the internal elections in Namibia that South Africa will hold Dec. 4 to 8. The move seemed designed to strengthen South Africa politically and militarily in the key Ovambo region, where African guerrillas have fought a low-level insurgency for nearly a decade.

Pretoria's announcement last month that it would go ahead with the unilateral elections in defiance of the United Nations and the Western negotiating group triggered the african calls for action on sanctions in the Security Council.

In another sign that Pretoria is determined to go ahead with the elections, invitations have been sent to a number of Americans, including major foundation heads jurists and journalists, asking them to go to Windhoek, the provincial capital of Namibia, to observe the elections.

Americans invited include strong critics of apartheid, South Africa's system of racial segregation. Millard Arnold, a Washington attorney, who directs the South Africa program of the Lawyers' Committee for Civil Rights Under Law, received an invitation although he has been twice refused visas to go to South Africa.

John Knowles, president of the Rockefeller Foundation, quickly rejected his invitation, in part because of fears that his presence would be used by the South Africans to claim international acceptance of the election results.

"They clearly haven't been reading my speeches and they certainly don't know anything about the Rockefeller Foundation and its history of commitment to racial equality," Knowles said in confirming his rejection of the invitation.

McGeorge Bundy, head of the Ford Foundation, also turned down an invitation but his office gave no reason for Bundy's refusal.

A South African Embassy spokesman in Washington said yesterday that approximately 100 persons from various countries have been invited by the South African-appointed administrator general of the territory, Judge Martinus Steyn, but the spokesman could not say if any had been accepted yet.

The embassy declined to comment on the reports of major increases in South African force levels in Namibia, previously reported to include at least 20,000 South African soldiers. The spokesman would say only that the military would put down any attempts by the guerrillas of the Southwest African People's Organization (SWAPO), which is based in Angola, and its allies to disrupt the elections.

Diplomatic sources said that troop increases could also be aimed at reinforcing the impression that the South Africans and their electoral allies in the territory in the future.

Angola's ambassador to Belgium, Luis de Almeida, told a news conference in Brussels Tuesday that Angolan President Agostinho Neto has mobilized a 200,000 strong militia force as well as the country's 31,000-man standing army because it fears that the build-up in Ovamboland is a prelude to a South African attack into Angola to hit at SWAPO.

Asked about the Cuban forces based in Angola since the 1976 civil war, de Almeida said Angola would call on Cuba and other friendly countries if necessary for self-defense.

After accepting this spring U.S. plan for Namibia elections that would have included SWAPO, South Africa struck into Angola and inflicted heavy losses on SWAPO and its installations, South Africa later withdrew its acceptance of the U.N. plan.

The Western negotiating team hopes that South African President Pieter Betha will agree to go ahead with U.S. supervised elections next summer once the internal elections, which the Carter administration has already described as null and void, are out of the way. The negotiating efforts at the United Nations yesterday continued to center on a draft resolution written by India.