The United States warned yesterday that the presence of 27,000 Cuban military personnel and civilian advisers in Africa could threaten movement toward resumption of normal U.S.-Cuban relations.

State Department spokesman Hoding Carter III said a new study by the administration had detected a major buildup of Cuban presence in Africa, including 23,000 Cubans - 19,000 of them military - in Angola.

That, he said, contradicted promises by President Fidel Castro's government to reduce its forces in Angola. Instead, according to the study, there has been a 20 per cent increase of Cuban personnel in the former Portuguese colony, whose Marxist government is battling insurgents.

"We believe the presence of large numbers of Cubans in Africa is bound to have an unsettling effect and is a threat to peace in Africa," Carter said.

Referring to the Carter administration's steps to improve ties with Cuba after a 16-year break, the State Department spokesman said the situation in Africa "will have an impact on peace and even the possibility of nor-normalizing relations."

He noted that the two countries had opened "interest sections" to represent each other diplomatically in Havana and Washington on Sept. 1. But, Carter added, "in light of the military activity we have gone as far as we can go at this time."

Revelation of the new estimates on Cuban forces, which are contained in the study by the National Security Council, marked a new escalation in the administration's public expressions of alarm about Cuban interference in Africa.

On Monday, the State Department said the number of Cubans in Ethiopia, which is embroiled in a conflict with Somalia over disputed territory, had increased to approximately 400 military advisers and 150 medical personnel.

The initial impact of Carter's statement appeared to be one of confusion within the State Department. Sources in the bureaus dealing with African and Latin American affairs said, at first, that they had not seen the NSC report and that its figures were, in several instances, higher than those they had been using.

Department spokesmen later explained that the NSC report was based on newer intelligence. They added that, at the time Carter made his statement, the report had not been fully distributed to those personnel dealing with Cuban and African matters.

The spokesmen refused to say what information and methods of computation had been used to arrive at the estimates in the NSC report. But, they added, the administration considers them an accurate and reliable gauge of the Cuban presence in Africa.

Although Carter pointed out that the intelligence study did not dispute the desirability of improving U.S.-Cuban ties, his statement did seem to signal a toughening of Washington's approach to dealing with Castro.

Administration officials have said consistently that the question of Cuban forces in Africa is one of three problems - together with settlement of U.S. financial claims for expropriated property and the human-rights situation within Cuba - that must be resolved before relations with Havana can be restored.

Until yesterday, though, the administration had tended to play down these problems and emphasize the progress that has been made toward improved relations through such means as the opening of the interest sections, the reaching of an agreement on fishing zones and exploring the possibilities of resumed trade.

In fact, State Department and White House sources have repeatedly cited the climate of improved relations with Cuba as one of the administration's major successes in the Latin American area.

The NSC report gave this breakdown of estimated Cuban strength in 16 African countries: Angola, 23,000, [WORD ILLEGIBLE] 19,000 military; Ethiopia, 400 military and 150 monmilitary; Uganda, an unconfirmed figure of 25 military; Sierre Leone, 120 to 125 military; Tanzania, 350 to 500, mostly technicians, and Mozambique, 650 to 750, including 150 technicians.

In other countries, the report said, the figures are: Madagascar, about 30 military; Libya, 100 to 125 military; Guinea, 300 to 500, mostly military; Guinea Bissau, 100 to 200, two-thirds of them military; Equatorial Guinea, 300 to 400, roughly half military; Congo, 400 to 500, about 300 of which are military; Cape Verde, 10 to 15 medical personnel; Begin; 10 to 20 security advisers; and Algeria, 35 medical personnel.

Somalia announced last weekend it is expelling the Cuban and Soviet advisers in that country.