Cuba announced today that its foreign minister, Roberto Robaina, stepped down and was replaced by a 34-year-old aide to President Fidel Castro. Robaina has played a key role in strengthening ties between Cuba and the outside world, and his unexpected departure is seen as one of the most significant leadership changes in Havana in years.

The announcement that Robaina, foreign minister since 1993, had stepped down was carried on the front page of Granma, the official newspaper of Cuba's ruling Communist Party. Robaina, 43, will be replaced by Felipe Perez Roque, a top aide to Castro who often accompanies the 72-year-old leader to meetings and on trips.

The reasons for Robaina's removal were unclear, and official media gave no hint of his plans. Some observers speculated that the departing foreign minister had not necessarily fallen out of favor, noting that his predecessor, Ricardo Alarcon, went on to be president of the National Assembly and plays a key role in domestic and foreign affairs.

Others, however, contended that Robaina might be paying for international criticism of tough new measures implemented by the Castro government to crack down on internal political opponents, particularly those suspected of having ties to the United States.

"Perez is a true Castro loyalist and a hard-liner, and his appointment could very well be part of the overall retrenchment that is going on within the government here," said one foreign observer in Cuba.

The Granma announcement could be read as implicitly critical of Robaina. It said that Castro decided on the change "taking into account the current complexity of the tense international situation, its growing importance for the future of our country and of the world and the need for deeper, more rigorous, more systematic and more demanding work in this area."

The statement also noted the close relationship between Castro and Perez, who is sometimes referred to as "the prince" and "Fidel's son." The newspaper said that "like few others, he is familiar with the ideas and thinking of Fidel."

"One would have to assume that Castro will play a more active role in the formulation of foreign policy given the nature of the appointment," said Wayne Smith, a senior fellow at the Center for International Policy in Washington and former chief of the U.S. interests section in Cuba.

Smith described Robaina as an effective foreign minister, particularly in building international opposition to the Helms-Burton Act, the 1996 law that tightened the unilateral U.S. trade embargo against Cuba. "He really traveled around and used Helms-Burton very effectively against the U.S. to a point where the U.S. was more isolated by it than Cuba," said Smith, a critic of the law.

Robaina is expected to retain his position in the Politburo, the highest political body in the Caribbean country of 11 million people. In addition to his other duties, Perez is a deputy in the National Assembly and a member of the Council of State. He also belongs to the Communist Party's Central Committee.