Many are wondering whether the 80-year-old Castro, a living symbol of revolution for many in the developing world, will guide the Cold War-era group in its search for relevance at a time of trade deals and globalization.
Castro ceded Cuba's leadership to his 75-year-old brother Defense Minister Raul Castro and a handful of other top government officials after emergency intestinal surgery in July. While reports of his recovery have raised expectations of a return to power, Raul remains acting president.
In his pajamas, Castro met with his close friend and political ally Venezuelan President Hugo Chavez on Thursday, but there was no sign he would be well enough to participate in the two days of sessions with more than 50 heads of state and government.
After the visit, Chavez said the Cuban leader was walking and singing and "almost well enough to play baseball." Still, even Chavez didn't say whether he thought Castro would make a formal appearance.
Later Thursday, Raul Castro presided over the Group of 15 developing nations on the sidelines of the summit, representing his nation at an international gathering for the first time as acting president.
But Chavez stole the show, saying Fidel Castro had given him permission to speak longer because Raul wouldn't talk much. Raul Castro seemed to take the jab in good spirits, giving Chavez a bear hug after the meeting.
Trading the green fatigues he's long worn as Cuba's defense minister for a dark suit, Raul Castro briefly praised Iran and other developing nations for trying to create "a better, more just world."
The U.N. has demanded Iran suspend uranium enrichment amid concerns it could be used for nuclear weapons. Iran insists the enrichment is aimed solely at producing electricity.
"Iran is under threat; there are plans to invade Iran. Hopefully it won't happen, but we are with you," Chavez told Iranian President Mahmoud Ahmadinejad.
"Under any scenario, we are with you just like we are with Cuba," Chavez added. "If the United States invades Cuba, blood will run ... We will not have our arms crossed while bombs are falling in Havana or they carry Raul off in a plane."
It was not clear what Chavez could actually do to help. In the past, he's vowed to cut off Venezuelan supplies of oil to the U.S. if it invades Cuba.
Ahmadinejad gave a relatively mild speech, urging his fellow leaders to work together to achieve their full potential. Then Chavez took over, saying he didn't want to leave Havana without a statement reflecting resounding support for the Iranian, Cuban and Palestinian people.
Nuclear proliferation and Middle East violence were also major topics; some diplomats said the developing world must unite to demand the creation of a Palestinian state. Lebanese Foreign Minister Fawzi Salloukh said the 118-nation Nonaligned Movement would consider a resolution to "condemn Israel for the hideous war waged against Lebanon."
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