The State Department cautioned Cuba yesterday that an expansion of its new military advisory group in Ethiopia into a major force would jeopardize the Carter administration's new thaw in relations with Havana.

Official U.S. information confirms that '50 or so Cuban military technicians are in Ethiopia," said State Department spokesman Hodding Carter III.

If the Cuban advisers turn out to be the vanguard of "a large number of Cuban troops," Carter said, "it would be a very serious development."

The United States has nothing to confirm such an intention, he said, but "we have already made clear that Cuba's intervention in Africa is an activity that could impede the improvement of relations" with the United States.

Washington Post correspondent David B. Ottaway on Wednesday reported the arrival of about 50 Cuban military advisers in Ethiopia about two weeks ago, in a despatch from Mogataghu, Sudan. He reported that Western diplomatic sources there regard the technicians as the advance group for about 400 to 500 Cubans to be sent to Ethiopia in the next few weeks.

The State Department has "no formation" on Cuban President Fidel Castro's intentions about sending more technicians "or combat troops" to Ethiopia, Carter said. Other reports of large numbers of Cuban troops being sent to African nations have not been borne out, he noted. But he said the United States "will be watching closely this development and any other Cuban activity in Africa."

Numerous steps toward normalizing relations between Cuba and the United States have been taken by the Carter administration, in significant departure from the policy of former Secretary of State Henry A. Kissinger.

Kissinger's policy made the withdrawal of Cuban combat forces from Angola the prerequisite for improving ties between Havana and Washington, severed in 1961 after Castro was in power. An estimated 10,000 to 15,000 Cuban troops are still in Angola, U.S. officials say.

Any new, large-scale Cuban intervention in Africa would confound administration policy, which has opened up visits to Cuba by sports teams, American cruise ship travel there and, last month, the first visit of a senior State Department official of Havana in 15 years.

In April the United States concluded a maritime and fisheries agreement with Cuba, and legislation is pending to lift part of the U.S. embargo on trade with Cuba.

Cuba's involvement in Ethiopia comes on the heels of Ethiopia's cutoff, a month ago, of a generation of close military ties with the United States. Ethiopia's Marxist military regime has switched its military dependence to the Soviet Union.

U.S. officials believe that Cuban military advisers may be in Ethiopia as surrogates for the Soviet Union, to try to help in the tangled maze there. Two separatist wars are under way in Ethiopia, in northern Eritrea and in the eastern Ogaden region on the Somalian border, with rivals Ethiopia and Somalia now both Soviet clients.

Andrew Young, U.S. ambassador to the United Nations, touched off a major controversy at the outset of the Carter administration when he said that "there's a sense in which the Cubans bring a certain stability and order" to Angola.

Young was charged with condoning the continued presence of Cuban troops in Angola. He denied that, and President Carter said he agreed that in one sense the Cuban presence in Angola "obviously stabilized the situation" for that nation's Marxist government. But the President said "the Cubans ought to withdraw their forces from Africa."

In South Bend, Ind., on Sunday, Carter criticized "the Cuban intervention in Angloa" as an unacceptable example of the extension of Soviet power "through the use of a client state's military force . . ."

A State Department official said yesterday that the United States recently told Cuba it wants to discuss the whole range of Cuban activity in Africa and that Cuba indicated it would do so. But the talks have not yet occurred.

The United States and Cuba have been moving toward the opening of diplomatic "interest sections" in each nation instead of working through the embassies of third nations. Many U.S. officials regard it as improbably that Castro is liekly to comfound this policy and other Cuban relationships by a major new venture in Africa, but they are not sure. U.S. officials estimate that Cuba has 2,000 to 3,000 advisers in about 10 African nations apart from Angola.