The movement loyal to anti-US cleric Moqtada al-Sadr in Iraq on Saturday released a man it identified as a captive former American soldier, but US officials declined to confirm the man's identity.
The man was named by followers of Sadr as 59-year-old army sergeant Randy Michael Hills, and Sadrist officials said he was freed in a humanitarian gesture and handed over to the United Nations mission in Iraq (UNAMI).
The UN said he was an American citizen, but provided no further details about the man's identity.
Two senior lawmakers loyal to Sadr "handed over to UNAMI in the evening of Saturday 17 March an American citizen whom they said has been in detention for about nine months by an Iraqi armed faction," UN spokesperson Radhia Achouri said.
"UNAMI is currently in contact with the US embassy in Baghdad to follow up on the matter," she added, but did not elaborate.
An official in Sadr's headquarters in the holy Shia city of Najaf said the man, whom he described as a "soldier," was captured on June 18, 2011.
"He was released for humanitarian reasons," said the official, who spoke on condition of anonymity. "There was no direct or indirect deal or negotiations with Americans or other sides."
The official and Maha al-Duri, a Sadrist MP, said the man had taken part in battles between the US army and Sadr's now deactivated Mahdi Army militia in 2004.
The US embassy declined to immediately confirm his identity, and the Sadrist claims appear to directly contradict US officials' account that the remains of the last soldier unaccounted for from the war in Iraq were handed over last month.
"We have heard the reports and we are attempting to verify the information," a US embassy spokesperson said.
The United States withdrew its military forces from Iraq in December after leading the 2003 invasion that ousted now-executed dictator Saddam Hussein.
Just 157 soldiers remain under the charge of the US embassy, along with a marine detachment responsible for the mission's security.