The Reagan administration said yesterday that South Africa's plan for an interim government in Namibia has "no standing" and insisted that it will have "no effect" on the international negotiating process aimed at bringing Namibia to independence.

A statement by State Department spokesman Ed Djerejian was briefer and much less blunt than confidential diplomatic protests made to South Africa earlier this week by the United States and other western "contact group" countries, according to official sources.

State Department and diplomatic sources said the earlier diplomatic statements told South Africa that its interim government plan is "null and void." An official of one of the western "contact group" countries working to arrange Namibian independence said South Africa also was told its action is considered "illegal."

State Department officials, while conceding that an interim regime in Namibia could complicate the drive for internationally recognized independence, said South Africa's announcement does not "pull the rug" from under the U.N.-sponsored independence plan.

"In practical terms, we doubt [that the interim regime] will last very long," said an official, noting that a similar arrangement had been attempted in 1978 but later collapsed. "We don't like it, but we think it will go away."

I. William Zartman, a professor at Johns Hopkins School of Advanced International Studies and a specialist on the Namibia negotiations, said the South African manuever "complicates tremendously" the U.S.-led international diplomacy. "It just throws in a new player" and "adds another layer of issues" involving the new government and a possible constitution, he said.

"Once again we are being bamboozled by the South Africans," Zartman said. "We keep on talking as if there is just one last thing" in the way of Namibian independence and a regional solution, he said, but "the South Africans keep on pulling out one last thing. They are nowhere near making a decision."

Experts said that a South African "internal solution" for Namibia had long been feared and that any move in this direction could have serious consequences.

Secretary of State George P. Shultz, in a speech Tuesday appealing for support of U.S. policy in South Africa, said that "our diplomacy, in concert with key western allies, has brought Namibia closer to independence than ever before." Shultz said that "agreement on a timetable for Cuban troop withdrawal from Angola is the one issue remaining in the overall settlement package" for Namibia.

Shultz voiced Tuesday, and the State Department repeated yesterday, the U.S. commitment to U.N. Security Council Resolution 435, under which the Namibia negotiations have proceeded, as "the only internationally acceptable basis for a solution."

The State Department, referring to Resolution 435, said yesterday: "The South African government has informed us that it intends to respect its international obligations and commitments. We expect South Africa to keep its word."

State Department officials said that, under the new South Africa plan, the South Africa-appointed administrator general in Namibia will remain in his post as the ultimate authority in Namibian affairs. Because of this, the interim regime will bring "no change" in the basic situation, an official said.

The next major step toward Namibian independence, according to the State Department, is the withdrawal of Cuban troops from neighboring Angola. "We are waiting for answers" to U.S. proposals presented to Angola on this issue. The withdrawal this week of the last South African troops from Angola "can only help," the official said.