The Reagan administration yesterday rejected again South Africa's plan for an interim government in Namibia as "null and void" and reiterated that South Africa remains an active participant in international negotiations seeking a formula for Namibian independence.

Elaboration on the administration position first enunciated Thursday came in a statement by State Department spokesman Bernard Kalb and at a briefing by Chester A. Crocker, assistant secretary of state for African affairs.

The net effect was denial that South Africa has derailed the Namibia negotiating process or is seeking to delay it by creating new obstacles. The Pretoria government Thursday announced its intention to establish an internal administration empowered to draft a constitution and exercise limited government powers in the disputed territory.

"We note that the announcement . . . affirms South Africa's intention to pursue the current negotiating effort aimed at achieving independence under the settlement plan" outlined in U.N. Security Council Resolution 435, Kalb's statement said.

"It is the U.S. position that these negotiations involving Namibian independence and Cuban troop withdrawal from Angola offer the best prospect for a settlement . . . ," it said.

It added that "any purported transfer of power that might take place now or in the future to bodies established in Namibia by South Africa is null and void. Such institutions will have no standing. We have not recognized them in the past and will not do so now."

Asked whether South Africa had broken its word to the United States, Crocker said Pretoria's promise to continue negotiations was "a recommitment" to the U.N. process. "We look at the rest as without effect and therefore irrelevant," he added.

Crocker noted that South Africa pursued a similar arrangement in Namibia from 1978 to 1983 that had never been recognized by other governments and that ultimately collapsed. Despite that, he said, independence talks had continued throughout that period.

South African Ambassador Bernardus Fourie also said his government's action "has nothing to do with negotiating an independence settlement. It was purely an internal arrangement that is intended to give the political parties within Namibia some experience in running their own affairs pending a settlement."

The administration's statements brushed aside the fact that the United States and other western countries made a strong effort to dissuade South Africa from taking unilateral action in Namibia. Instead, Crocker said, the situation should be viewed in terms of other interrelated actions in recent days.

He noted that, in mid-March, the United States made proposals to South Africa and Angola for an "overall package agreement" aimed at bringing about Namibian independence and Cuban withdrawal.

He said that both sides are expected to reply shortly and that the United States sees "room for movement."