After delaying for a year, South Africa will complete its pledged troop withdrawal from Angola before the end of this week, Foreign Minister Roelof F. (Pik) Botha announced today.

It will mean that for the first time since 1981, there will be no South Africans in Angola.

In announcing their unilateral withdrawal, the South Africans abandoned their previous insistence on having the border between Angola and Namibia patrolled jointly by South African and Angolan troops. Angola consistently had refused to consider this idea, or the continuation of a monitoring committee that had supervised the South African withdrawal back to the town of Ngiva, about 25 miles north of the frontier.

Botha said that South Africa had delayed the withdrawal because guerrillas seeking Namibian independence, the South-West Africa People's Organization (SWAPO), had been carrying out incursions during the rainy season, which has just ended.

Botha warned in announcing the withdrawal that South Africa would feel free to send troops back across the border into Angola if incursions by SWAPO resume.

"The South African government trusts that its decision to complete the disengagement process will enhance the prospects for peace in the region and will, in particular, be conducive to withdrawal of the Cubans from Angola," the announcement said. "Such a development would open the way to the realization of . . . the peaceful resolution of the problems of the region, including the question of independence of South-West Africa-Namibia."

The announcement was welcomed in Washington, but it could lead to new tensions between the United States and the Botha government over South Africa's approach to the Namibia-Angola issue.

According to informed sources here, President Pieter W. Botha is expected to announce Thursday that he is prepared in principle to appoint an "interim" Namibian government made up of the disputed territory's internal political parties. It is predicted that President Botha will make this announcement in the course of a detailed response to a speech that Secretary of State George P. Shultz is to make to the National Press Club Tuesday.

The apparent revival of earlier South African attempts to put together an international, or regional, solution to the drawn-out Namibian dispute appears to be worrying the U.S.-led "contact group" of five western nations that has acted as the mediator in the dispute for the past seven years.

["We are quite pleased about the disengagement," a State Department official said in Washington. But he indicated that the pullout was a separate issue from the question of South African moves toward independence for Namibia outside the provisions of U.N. Security Council Resolution 435. The resolution mandates establishment of a U.N. peace-keeping force in Namibia and the withdrawal of all but 1,500 South African troops.]

U.S. Ambassador Herman Nickel and the ambassadors of Britain, West Germany and Canada called on Foreign Minister Botha today. They made no statements afterward, but sources said that they had emphasized their governments' commitment to an internally supervised settlement in Namibia as specified in the U.N. resolution.

France suspended participation in the contact group last year because of the lack of progress toward independence for Namibia.

The purpose of establishing a new "interim" government is not clear. A previous one set up by South Africa with a group of internal parties called the Democratic Turnhalle Alliance proved a failure and was disbanded by President Botha two years ago.

One theory is that South Africa may be deliberately raising the threat of unilateral action as a way of putting additional pressure on Angola for the withdrawal of Cuban troops from its territory, which remains the main stumbling block in the way of a Namibia settlement.

Whatever the reason, it means that after months of immobility, during which the Reagan administration's policy of "constructive engagement" with South Africa has come under increasing criticism in the United States because of its perceived failure to produce results, the Namibia issue is moving into the spotlight again.

Three weeks ago Chester A. Crocker, assistant secretary of state for African affairs, announced that he had presented a new package of proposals to the Angolan and South African governments aimed at breaking the deadlock over Namibian independence.

Now South Africa seems to be working up a new plan of its own that includes setting up a group of internal parties called the Multiparty Conference as an "interim" government.

Crocker gave no details of his package, except to indicate that it included a shift to something less than the total withdrawal of Cuban troops from Angola, which the U.S. had demanded earlier as a condition of Namibian independence.

This seemed to indicate some sort of phased withdrawal of a kind that Angola has said it is willing to contemplate, but which South Africa is reluctant to accept.

Crocker also has spoken of the need to respect the territorial integrity of Angola and Namibia. This seemed to be a reference to South Africa's continued military presence in Angola, despite a withdrawal agreement that he helped negotiate in February 1984 and that was supposed to have been completed within three months.