The movement loyal to anti-US cleric Moqtada al-Sadr in Iraq on Saturday handed over to the United Nations a man it identified as a captive former American soldier who had been held for nine months.
The man, phonetically named by Sadrist officials as 59-year-old ex-army sergeant Randy Michael Hills, appeared wearing a US military uniform at a news conference in Baghdad alongside two senior Sadr movement politicians.
The US embassy confirmed the man transferred to the UN mission in Iraq (UNAMI) was a US citizen, but provided no further details about his identity.
"UNAMI has transferred a US citizen to the US Embassy in Baghdad, which is providing all necessary consular assistance to the US citizen," an embassy spokesman said.
"Due to privacy considerations, we are unable to provide additional information."
Earlier, UN spokeswoman Radhia Achouri said parliament deputy speaker Qusay al-Sohail and MP Maha al-Duri "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."
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An official in Sadr's headquarters in the holy Shiite 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 Duri said the man had taken part in battles between the US army and Sadr's now deactivated Mahdi Army militia in 2004.
US officials have said that the remains of the last soldier unaccounted for from the war in Iraq were handed over last month.
American civilian contractors and interpreters often wore US military uniform or clothing approximating military fatigues while US forces were stationed in Iraq.
Washington 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.