Syria's Assad compares Castro's anti-US struggles to his own
"The great leader Fidel Castro led his people's and his country's struggle against imperialism and hegemony for decades," said Assad - whose regime 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.
The US and the EU imposed a series of sanctions on Syria since the war started in March 2011 with a massive regime crackdown on anti-government protests.
Most of Assad's inner circle and top military aides are the subjects of US and EU travel bans and assets freezes.
Meanwhile Cuba has maintained strong support for the Assad regime, and reportedly even sent military officials to Syria to provide logistic support for the regime's war efforts.
Although Castro increasingly retired from public life after 2006, he has been upheld as a source of ideological inspiration by the Assad regime as the war overwhelms the country.
Speaking in July this year during a press conference called for stronger bilateral ties with Cuba, Assad described Castro as an "icon of independence struggle".
Agencies contributed to this report.