Prime Minister Robert Mugabe today cautioned U.S. Deputy Secretary of State William Clark against pursuing changes in a U.N. plan for the independence of Namibia (South-West Africa).

Clark arrived here from Windhoek, the Namibian capital, where he said South Africa was making a "reasonable attempt" to comply with that U.S.-initiated plan, formulated in a U.N. resolution.

A statement by the Zimbabwwean government, issued after 90 minutes of talks, said Mugabe "expressed concern at any suggestion to deviate from Resolution 435. He urged all parties to work within the U.N. framework for the implementation" of the resolution.

The United States has been pressing for changes in the plan to satisfy demands for safeguards for the white minority by South Africa, which controls Namibia and has refused to grant independence.

The government statement did not characterize the talks, but a member of the Zimbabwean delegation was quoted as saying, "They told us nothing new." Clark simply said he had "an enjoyable time" and the talks were "very helpful." He flew to Paris tonight after visiting South Africa as well as Namibia and Zimbabwe.

Some Zimbabwean officials believe that Clark's stopover was a "courtesy" to black Africa and that the real negotiations were with South Africa.

The talks are the first high-level discussions with a black African government by the Reagan administration since it began to woo South Africa with offers of friendship in return for a Namibian settlement.

Mugabe has generally adopted a wait-and-see attitude toward the policies of the new U.S. administration, although he has criticized its efforts to link a Namibian settlement to withdrawal of Cuban troops from neighboring Angola.

Mugabe has also criticized U.S. suggestions for constitutional safeguards for the 100,000 whites in Namibia as part of a settlement. However, a veteran of years of war and negotiations to obtain independence for Zimbabwe, Mugabe made similar constitutional compromises at the British-sponsored settlement talks in London in 1979 that led to black rule here.

Washington Post correspondent Caryle Murphy reported from Johannesburg that Clark, in characterizing South Africa's "reasonable attempt" to give independence to Namibia, added that Washington has not yet decided whether to pursue the present Western diplomatic initiative to end the bush war and to establish black majority rule there.

Clark said that on his return to the United States he would brief U.s.Secretary of State Alexander M. Haig Jr. and the president on the "facts and opinions" he is gathering. Haig is currently touring Southeast Asia.

"At that time they will make a judgment of what commitment the South African government has made in moving the independence process off dead center," he said.

When asked it the United States was considering the option of pulling out of the effort altogether, Clark replied, "We are not going to recommend going forward with something we regard as unfair or unworkable." But he added that no decision had been made yet and that this was not intended as an ultimatum to the South Africans.

Clark, accompanied by Assistant Secretary of State for African Affairs Chester Crocker and Assistant Secretary of State for International Organization Affairs Elliot Abrams, is attempting to establish whether it is possible for the United States to go ahead with the Namibian settlement plan worked out by the previous administration and four U.S. allies.

South Africa's administration of the vast semi-desert territory, originally mandated by the old League of Nations at the end of World War I, has been declared invalid by the United Nations. But South Africa has refused to leave the territory where it is engaged in a low-level bush war with guerillas of the South-West Africa People's Organization (SWAPO).

Clark said that any specific proposals the United States would make on the settlement plan would be presented to Britain, France, West Germany and Canada.

The South African demands, reportedly laid out in a letter to Haig after Foreign Minister Pik Botha's visit to Washington in mid-May, and presumably being discussed on this trip, include requests for constitutional guarantees for the 110,000 whites living in Namibia, for guarantees that future elections would be held and for a larger South African role in running the election with the U.N. officials.