Amir Hekmati, a former US marine, was arrested in Tehran in August 2011 for his alleged links with the CIA. He was accused of espionage by Iran.
Hekmati, 29, said his confession which was aired on Iranian state television was forced and used to fabricate him in the trial, the Guardian reported citing a letter written by Hekmati.
The letter, addressed to US Secretary of State John Kerry, was smuggled out of jail and was obtained by the Guardian.
"For over two years I have been held on false charges based solely on confessions obtained by force, threats, miserable prison conditions and prolonged periods of solitary confinement," Hekmati wrote.
The authorities in Iran has told Hekmati's state-appointed lawyer that he would only be released if two Iranian prisoners held in foreign jails were freed, according to the letter.
"This is part of a propaganda and hostage-taking effort by Iranian intelligence to secure the release of Iranians abroad being held on security-related charges," the letter says.
Hekmati was sentenced to death in January 2012 by an Iranian court. The verdict was later cancelled by a higher court. He is still awaiting a retrial.
John Kerry last month urged Tehran to release Hekmati from prison on the second anniversary of his arrest, saying Washington was "deeply concerned" about his detention.
--IANS (Posted on 11-09-2013)