BY JUAN O. TAMAYO

  Cuba is suffering through its worst economic crisis since the collapse of the Soviet Union, but significant improvements are impossible as long as President Fidel Castro lives, a top defector said Thursday.

  ''We are all waiting for the physical disappearance of Fidel,'' said Alcibíades Hidalgo, a former Cuban ambassador to the United Nations and personal secretary to
  Castro's brother, armed forces chief Raul Castro.

  Hidalgo, the highest-ranking defector to arrive in Miami in recent years, said that the worst economic situation the island has faced since the early 1990s has seen a
  ``significant worsening over the last few months.''

  Tourism and prices for key Cuban exports such as nickel, sugar and tobacco are all down, Hidalgo told a large audience at the University of Miami's Institute for Cuban and Cuban-American Studies.

  But no changes are possible until Fidel Castro leaves power because he rules Cuba single-handedly, with little or no input from supposedly powerful institutions like the Cuban Communist Party, said Hidalgo, who arrived last month.

  The only other people who hold real power are the senior military officers who command the armed forces and the Interior Ministry, in charge of domestic security,
  Hidalgo added.

  Castro's death will cause ''an enormous commotion,'' he said, adding that he would not rule out the possibility of a popular uprising.

  ''For me, Castroism without Fidel Castro will not exist,'' he said in between sometimes unfriendly questions from audience members who scolded him for his past support of Castro and wondered if he was a Cuban spy.

  Hidalgo said he had no doubt that Castro has sheltered foreign terrorists, such as members of Spain's ETA Basque separatist movement, and produced chemical
  weapons with assistance from the Soviet military.

  But he has no evidence, he added, that Cuba has experimented with biological weapons or established links with Osama bin Laden's al Qaeda terror network. ''Maybe bin Laden would want nothing to do with an atheist Marxist,'' he joked.

  Hidalgo said he was fired as ambassador to the United Nations and sent into political oblivion in 1993 for complaining that too many spies were being assigned to the Cuban diplomatic mission to the United Nations.

  Asked about specific spy operations out of the United Nations, Hidalgo first said he knew little about them, then added, "Besides, I am writing a book.''