Cuba Fidel Castro birthday
Cuba's former president Fidel Castro (R), Bolivia's President Evo Morales and Venezuela's President Nicolas Maduro sit together in a van in Havana, CubaReuters

Former Cuban leader Fidel Castro has asked the US to repay "millions of dollars it owes" the country, as he celebrates his 89th birthday.

Following improved ties between the two countries, the former Cuban president said the money owed stemmed from the 50-year-long trade embargo.

"Cuba is owed compensation equivalent to damages, which total many millions of dollars, as our country has stated with irrefutable arguments and data in all of its speeches at the United Nations," he wrote in an open letter to the nation which was published in the local media. The brief letter did not specify the exact amount due to Cuba.

Since he passed on power to his brother Raul in 2006, he has remained mostly out of public life but regularly makes comments in various Communist forums.

"Writing is a way to be useful, if you keep in mind that we poor humans must be more and better educated in the face of the incredible ignorance that surrounds us all, except for researchers who use science to seek a satisfactory answer," he wrote in the letter carried by the Communist Party newspaper Granma to mark his birthday.

Bolivia's populist President Evo Morales and Venezuelan President Nicolas Maduro are joining the celebrations.

Castro's latest remarks have come on the eve of the historic re-opening of Washington's embassy in Havana by US Secretary of State John Kerry after more than half a century of frosty US-Cuba relations.