"The great leader Fidel Castro led his people's and his country's struggle against imperialism and hegemony for decades," Assad, whose government is facing US sanctions, said in a message of condolences.
"His resistance became legendary and inspired leaders and people all over the world," he said in the letter addressed to Castro's brother Raul, who is president of Cuba.
"Cuba, a friendly country, was able thanks to its leaders to resist against the toughest sanctions and most unjust campaigns in our modern history," Assad said, referring to the US embargo on the island.
Most of Assad's inner circle and top military aides are the subjects of US travel bans and assets freezes.
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