The United States has decided that Japan must delay the start of trials of its first nuclear reprocessing plant, which Japan regards as vital to its future energy plans, U.S officials said yesterday.

As Japan's sole supplier of enriched uranium fuel for nuclear power stations, the United States retains the right to control reprocessing of the fuel. President Carter has called for an indefinite halt to all commercial reprocessing as part of his campaign against nuclear proliferation.

The dispute is critically timed for Premier Takeo Fukuda, whose party faces a difficult upper house election Sunday. Shelving the plant and its 400 employees expose Fukuda's ruling Conservative Party to charges of knuckling under to Washington. Fukuda had hoped to begin trials at the plant this month.

Japanese and U.S. scientists are working on a compromise on ways to run the $170-million, French-built plant that would produce a plutonium-uranium mixture that can not be used in weapons. Their report is expected to be completed next week. [TEXT OMITTED FRM SOURCE] Patriotic Front. The Front received a strong endorsement at this week's Organization of African Unity Meeting.

In Lusaka, Zambia, American and British negotiators postponed a meeting with key black Rhodesian nationalists, including representatives of the Patriotic Front.