The State Department said yesterday that a Western plan to bring the South African-controlled territory of Namibia to independence peacefully is back on the track and that a U.N. supervised settlement should be achieved next year.

Richard Moose, assistant secretary of state for African affairs, made the upbeat assessment in a breakfast meeting with reporters in which he admitted pessimism about the situation in Rhodesia, the other southern African area where the United States is attempting to bring about a peaceful transition to black majority rule.

The West has hoped that a peaceful solution to the Namibia problem could serve as a model for strife-torn Rhodesia. Failure, on the other hand, would most likely lead to an escalation of the low-key guerrilla war being waged by the Southwest Africa People's Organization (SWAPO) and the possibility of increased Soviet-Cuban involvement in the area.

There has also been mounting pressure for the United Nations to impose sanctions on South Africa, a move the West opposes, because of large investments there, and would probably veto, leading to further East-west polarization in Africa.

Moose called the Namibia negotiations "the most successful U.S. undertaking in Africa this year."

He said South Africa informed U.N. Secretary General Kurt Waldheim last week that it was ready to allow the world organization to go ahead with implementation of the Western plan for independence of Namibia, and called for elections by Sept. 30.

South Africa has ruled sparsely populated Namibia-about twice the size of California-as the territory of Southwest Africa for 58 years under a League of Nations mandate revoked by the United Nations in 1966.

The South African move should allow elections to establish black-majority rule by August or September, Moose said. The Western plan calls for the elctions to be held about seven months after implementation begins with the introduction of up to 7,500 U.N. peace-keeping forces to replace all but 1,500 of the estimated 25,000 South African troops in the territory.

South Africa, after originally accepting the western plan last spring, reneged in September after African nations brought pressure on SWAPO to go along with the proposals, then went ahead and held elections for a constitutent assembly this month that a South African-backed faction won. SWAPO BOYCOTTED THE ELECTIONS, WHICH WERE INTERNATIONALLY CONDEMNED.

OFFICIALS OF FIVE WESTERN NATIONS-THE UNITED STATES, BRITAIN, FRANCE, WEST GERMANY AND CANADA-HAVE BEEN MEETING THE LAST FEW DAYS TO DISCUSS THE SOUTH AFRICA RESPONSE.

EXPECTATIONS ARE THAT THE NEXT STEP WILL INVOLVE U.N. representative Martti Ahtisaari visiting Namibia to held talks with the South African administrator, Marthinus Steyn, on operational questions and logistics involved in the arrival of U.N. peace-keeping forces.

Don McHenry, U.S. ambassador to the United Nations and the key American official in the two-year-long Namibia negotiations, said in a telephone interview that the South African reply provided reason "to be fairly optimistic," especially when compared with the situation in the fall, when it appeared that South Africa might impose an internal settlement. He cautioned, however, that "it's not over yet."

On the question of Rhodesia, where warring factions have refused to negotiate, Moose was much more pessimistic. Asked about the statement by Sen. George McGovern (D-S.D.) that the Anglo-American peace effort there had failed and the United Sates should "disengage," Moose refused to go that far.

He added, however, that chances for a peaceful settlement are shrinking. His remarks were sprinkled with phrases such as "we gave it a fair run" and "we don't have great leverage there."

His main hope seemed to be that the escalating 6-year-old guerrilla war in Rhodesia could lead to a change in perceptions by the white-dominated government or by South Africa, its main supporter.

"Maybe be time will come when the parties want to negotiate," he said, adding that it is not yet time for the United States "to stand aside and let the sides fight it out."

One glimmer of White Rhodesian despair came from Salisbury yesterday when Rollo Hayman, the white co-minister of internal affairs and a close ally of Prime Minister Ian Smih, resigned and called for acceptance of the Anglo-American plan.