Cuban leader Fidel Castro says claims by US presidential hopeful John McCain that Cubans helped the North Vietnamese torture US POWs in the 1960s are lies.
Fidel Castro, not seen in public for 18 months, pens frequent articles
Mr Castro, writing in a state-run paper, said the claims were baseless.
Senator McCain retorted by saying to respond to one of world's "most brutal dictators" would be beneath him.
Mr McCain, a pilot held for five years in Vietnam, first made the remarks in his memoirs and repeated them last month while campaigning in Florida.
The support he gained from the Cuban exile community in Florida helped him win the state's Republican primary but may also have prompted the response from Fidel Castro, correspondents say.
"His accusations against internationalist Cuban revolutionaries...are completely unethical," Mr Castro wrote in the Granma newspaper on Monday.
"Let me remind you, Mr McCain. The commandments of the religion you practise prohibit lying," the article reads.
Speaking on 25 January, Mr McCain told supporters in Florida: "There's a person I want you to help me find when Cuba is free, and that's that Cuban that came to the prison camps of North Vietnam and tortured and killed my friends. We'll get him and bring him to justice, too."
Mr McCain, the front-runner for the Republican nomination, was shot down over North Vietnam in 1967.
Cold War allies
During his five years in captivity, he experienced frequent beatings and abuse which left him with limited movement in one arm.
He has said that while Cubans did not torture him, a couple of Cuban agents came to show the Vietnamese "some new interrogation techniques".
At the time, North Vietnam and Cuba were Cold war allies.
Out campaigning in Maryland on Monday Mr McCain declined to respond directly to Mr Castro's remarks.
"For me to respond to Fidel Castro who has oppressed and repressed his people and who is one of the most brutal dictators on Earth, for me to dignify any comments he might make is certainly beneath me," he said.
"It's a matter of record and you can ask several of the POWs who had direct contact with ... some thug that came to Hanoi with an underling assistant."