Tanzanian President Julius Nyerere told U.S. ambassador to the United Nation Andrew Young today that Washington should use its economic leverage on Rhodesia's white-minority government to help achieve agreement on the British plan for transition to black majority rule.

After an hour-long meeting in the Tanzanian capital, Nyerere said that since the Anglo-American plan to end Rhodesia's 11-year-old constitutional crisis has reached a deadlock, the U.S. role is to repeal the Byrd amendment - the controversial legislation that allows the United States to buy chrome from Rhodesia - and to ban other "invisible" trade relations with the sanctioned territory.

Today's session - believed to be the most important talk during Young's 10-day tour of Africa - did no resul in the anticipated boost for new U.S. involvement in the problems of southern Africa.

Nyerere stressed that a Rhodesian settlement is still exclusively in the hands of the British, indicating that the United States should play a suportive but low-key role. He said the negotiations between white government officials and four African nationalist leaders, which lasted for two months in Geneva last year, are not necessarily over.

"I believe that the next move in Rhodesia should be for the British and the nationalists to continue their talks until they have agreed as to the kind of change they would like to see in Rhodesia," he said.

Nyerere added, "It's not wise for talks to be given up simply because [Rhodesian Prime Minister Ian] Smith rejected the British proposals. I think essentially the agreement we seek is not between the British and Smith. Talks should continue between the British and the nationalists.

But at a joint press conference after the meeting at his oceanside home, Nyerere did not relay any signals that the five "front-line" presidents - of Botswana, Tanzania, Zambia, Mozambique and Angola - involved in the settlement effort were looking to the United States to lead the way in a new strategy.

Only when there is an agreement did Nyerere say he hoped the United States would take an active role in "seeing that Smith is out of the way" by tightening economic sanctions imposed by the United Nations after Rhodesia unilaterally declared its independence from Britain.

According to officials traveling with Young's delegation, Nyerere's statement today typified the feelings of the four other African leaders the new ambassador met on Zanzibar last week. On Friday and Saturday, Young held brief talks with the presidents of Somalia, Zambia, Rwanda and Burundi.

So far the message appears to be that black Africa wants the United States to maintain an interest in the problems of southern Africa but not to take over as the primary force in negotiating terms for majority rule in Rhodesia and Namibia.

Nyerere was the first to offer any indication of just what the next step for the United States could be, but this message could be something of a letdown to Young since the Carter administration has already endorsed these actions in principle, and they involve no significant new initiative.

The interview was another in a series of anticlimaxes during Young's much-heralded tour of Africa. The former Georgia congressman has long been a admirer of the Tanzanian leader and Young said the two have corresponded over the years.

"He's really been the teacher today and I've been the student," Young said, referring to Nyerere's Swahili title mwalimu, which translates as "teacher." "I've sat at his feet and learned from him."

Their two brief meetings Friday and today, their first face-to-face encounters have provided new contact and a point from which to begin strengthening U.S.-African relations. There is little evidence so far that the African leaders Young has met are interested in more active U.S. involvement in their problems.

Young is to leave Tanzania Monday for Nigeria, the last stop of his tour, where he is to attend an international art festival and hold further talks.