* Fidel Castro, Chavez were close allies
* Chavez provided oil, aid to Cuba
HAVANA, March 11 (Reuters) - Cuba's Fidel Castro praised the late Venezuelan President Hugo Chavez on Monday as a champion of the poor and said Cubans had lost their best friend ever, in his first comments on the death last week of his socialist ally.
Castro said the news, although not unexpected, had been a hard blow.
"On the 5th of March, in the afternoon hours, died the best friend the Cuban people had in their history," Castro wrote in a column published in Communist Party newspaper Granma.
"We have the honor of having shared with the Bolivarian leader the same ideals of social justice and of support for the exploited," said the 86-year-old Castro who led Cuba's 1959 revolution, ruled the country for 49 years and still plays a behind-the-scenes role.
"The poor are the poor in any part of the world," he said.
During Chavez' years in power, he and Castro forged a close personal and political relationship that resulted in extensive Venezuelan aid to the communist island and a shared strategy for promoting Latin American unity against U.S. influence in the region.
Chavez helped rescue Cuba from desperate economic times that followed the 1991 collapse of the Soviet Union, its former top ally, by providing two-thirds of its oil in a barter deal for the services of Cuban professionals, most of them doctors and nurses.
He also signed a number of joint ventures aimed at integrating the two countries' economies.
Chavez, 58, was diagnosed with cancer in the pelvic region in June 2011 by Cuban doctors and underwent four surgeries on the Caribbean island, which has an extensive medical system and provides free care to its people.
Except for a set of photographs, Chavez was never seen in public again following his last operation in December and he died on Tuesday in Caracas.
Castro said he had received a phone call via satellite notifying him of what he called "the bitter news."
"The significance of the phrase used was unmistakable. Although we knew the critical state of his health, the news hit us hard," wrote Castro, who resigned as Cuba's president five years ago because of his own health problems.
"I remembered the times he joked with me saying that when both of us finished our revolutionary work, he would invite me to spend time by the Arauca River in Venezuelan territory, which reminded him of the rest he never had," Castro said.
Raul Castro, who succeeded his older brother as Cuba's president, represented the island on Friday at Chavez' funeral.
Chavez' death has raised worries in Cuba that Venezuelan aid will cease to flow to the island.
His preferred successor, Nicolas Maduro, is favored to win an April election to replace Chavez and expected to continue his Cuba policies for the immediate future.
However, if more conservative opponent Henrique Capriles pulls off an upset victory, he has promised to put an end to Venezuela's oil largesse.
Castro closed his column by paraphrasing a famous quote from another late friend and revolutionary, Ernesto "Che" Guevara," the Argentine physician who fought alongside him in the Cuban revolution.
"Until victory always, unforgettable friend," Castro wrote of Chavez. (Reporting By Jeff Franks; editing by Christopher Wilson)