Obama told the Ladies in White, a group of wives and children of political prisoners, that he understood their struggle, in a letter dated March 10 but published online by the dissident organisation three days later.

"I fully understand the obstacles that ordinary Cubans face in exercising their rights," Obama wrote in English.

"As I have in the past, I will raise these issues directly with President Castro," Obama stressed.

The White House confirmed to media that the letter was authentic.     

When Obama sets foot in Havana on March 20, the White House imagines a "Berlin Wall moment" - a singular legacy-gilding event like Ronald Reagan's 1987 address before the Brandenburg Gate.

While Reagan sought to end the Cold War division of Europe, Obama hopes to symbolically "tear down" decades of Cold War antagonism across the narrow Florida Straits.

Obama will visit the island March 20 to 22 - the first visit by a US president since Calvin Coolidge in 1928, and a symbolically charged capstone to the rapprochement that he and Castro announced in December 2014.

Obama's Republican foes accuse him of betraying the cause of human rights in Cuba by engaging with the Castro regime, the Americas' only one-party Communist state.

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