Cuban President Fidel Castro has disclosed that Cuba stopped withdrawing its troops from Angola in April.

Word of Castro's remarks came one day after Cuba and the United States announced the most important step toward normalizing of their relations since the United States cut off diplomatic ties with the Communist nation 16 years ago.

The two countries announced Friday that they soon would begin an exchange of middle-level diplomats, a move the Carter administration stressed was only a preliminary move toward resumption of full diplomatic relations. One of the largest obstacles to restoration of full relations, White House press secretary Jody Powell said, is the continued presence of Cuban troops in Angola.

Castro's disclosure that the withdrawal of Cuban troops from the African nation has stopped came during a May 19 interview in Havana with ABC-TV newscaster Barbara Walters.

In the interview, which will be aired by ABC next Thursday night, Castro said his government halted the withdrawal because France and Morocco had intervened in the fighting against insurgents in Zaire's Shaba Province. French aircraft flew weapons and Moroccan soldiers into Zaire and ferried both Moroccan and Zairian soldiers into Shaba Province to put down an insurrection.

In an April interview with the Algerian magazine Afrique-Asie, Castro viewed the French-Moroccan action as a prelude to an invasion of Angola and warned that "any attack against Angola will be considered by us as an attack on Cuba."

Castro's comment in the TV interview came almost a year after he announced a phased withdrawal of Cuban troops from Angola. In June 1976 it was reported that Cuban troops were being returned home at the rate of 200 a week.

A Cuban official told visiting Rep. Jonathan Bingham (D-N.Y.) in February that half of the Cuban troops in Angola has been returned at that time. Former Secretary of State Henry Kissinger maintained that Cuba had 12,000 soldiers in Angola during the height of the Angolan fighting in early 1976, but Castro has told several foreign visitors recently that this figure under estimated Cuban strength there. Currently U.S. officials estimate there are 10,000 to 15,000 Cuban troops in Angola.

At the time he announced the phased withdrawal from Angola a year ago, Castro said the government there had asked Cuba to keep enough troops in the country to insure against "internal and external threats."

Zaire has repeatedly accused Cuba of training, supporting and leading the insurgents in Shaba Province, Castro has strongly denied it, calling the fighting there "a purely internal affair of Zaire."

The exchange of Cuban and U.S. diplomats will involve middle-level diplomats from each of the countries operating under the flags of other nations, the Cubans using the Czechoslovakian embassy in Washington and the Americans the Swiss embassy in Havana. Castro's comment in the television interview is expected to be used by opponents of normalization of U.S.-Cuba relations as an example of why relations should not go beyond the preliminary step without firm conditions.