Cuban President Fidel Castro suddenly withdrew from a U.N. summit and returned home, and Cuban officials indicated someone — possibly the United States — had offended the leader.

Ricardo Alarcon, president of Cuba's National Assembly, told The Associated Press after Castro's departure Thursday afternoon that Castro left because of "a situation that for a self-respecting country like Cuba, was unacceptable."

Alarcon, who took Castro's place as head of Cuba's delegation to the U.N. International Conference on Financing for Development, indicated that the incident involved the United States, but would not elaborate.

Alarcon wouldn't specify whether the Cuban leader's departure was related to the arrival about 30 minutes earlier of President Bush, or whether the United States had exerted pressure to avoid the two running into each other.

Bush's national security adviser, Condoleezza Rice, said earlier that Bush and Castro would not cross paths, and Alarcon said: "She knew why she was saying that."

"In the final analysis it is a problem with the United States. That doesn't mean that someone from the United States talked to us or asked us to do something," Alarcon said.

"We had to say something so people would understand that something had happened, something that forced Cuba to take this decision. We have acted with moderation and in a constructive spirit," he said.

Castro, who arrived in Monterrey on Wednesday night, addressed the conference Thursday morning, calling the international financial system "a gigantic casino" and lashing out at rich nations for blaming poverty on the developing world.

"You can't blame this tragedy on the poor countries. It wasn't they who conquered and looted entire continents for centuries, nor did they establish colonialism, nor did they reintroduce slavery, nor did they create modern imperialism," he said. "They were its victims."

According to a U.S. official who spoke on condition of anonymity, the U.S. delegation had been instructed to leave the designated U.S. seat when it was Castro's turn, and they did. No reporters were allowed into the session, and the television feed showed only the podium.

Castro, still addressing the gathered leaders, excused himself and said he had to return immediately to Cuba, citing only "a special situation created by my participation in this summit."

Mexican officials said they had no idea why Castro was leaving, as did his close friend Venezuelan President Hugo Chavez.

"I really don't know why Fidel is leaving today," Chavez said.