A man hangs a painting during an exhibition of visual artists in honor of the 90th birthday of Cuba's former President Fidel Castro in Havana, Cuba, August 13, 2016. [Photo/Agencies]

In his letter, Castro accompanied his thanks with reminiscences about his childhood and youth in eastern Cuba, describing the geology and plant life of the region where he grew up. He touched on his father's death shortly before his victory in overthrowing US-backed strongman Fulgencio Batista in 1959.

Castro also took the opportunity to criticize Obama, who appeared to anger the revolutionary leader with a March trip to Cuba in which he called for Cubans to look toward the future. A week after the trip, Castro wrote a sternly worded letter admonishing Obama to read up on Cuban history, and declaring that "we don't need the empire to give us anything".

In Saturday's letter, he attacked Obama for not apologizing to the Japanese people during a May trip to Hiroshima, describing his speech there as "lacking stature".