|Muqtada al-Sadr's Mahdi Army targeted US troops during the nine-year war in Iraq [GALLO/GETTY]
The United Nations Mission in Iraq says it has taken custody of a man who was described as a US citizen, who had been held by Iraqi fighters for about nine months.
US officials said on Saturday the man appeared to be a US contractor.
Radhia Achouri, a UN spokeswoman, did not identify the man, whom she said was staying at the UN compound in Baghdad's fortified Green Zone.
A spokesman for the US embassy in Baghdad also could not immediately identify the man.
Achouri said he was handed over by two Shia legislators who represent the followers of anti-American cleric Muqtada al-Sadr. She did not provide any details.
Abdul Hadi al-Mutairi, a senior Sadrist official, told The Associated Press news agency that the man was a US soldier named Michael Hill who was captured June 18.
Al-Mutairi said the man was wearing a US military uniform when he was delivered to the UN.
Several US officials said he was a private contractor, rather than an active-duty soldier, although he may have previously served in the US military.
Hundreds of thousands of contractors, both US and other nationalities, worked alongside US troops and in other support roles throughout the war.
Al-Mutairi said the man was released without any negotiation "as a goodwill initiative toward the American society and to [his] family".
He said the man, whom he said was married and had two sons, was treated well during his nine-month imprisonment, "even though he contributed in the battles in Sadr City and in Najaf".
Al-Sadr is the spiritual leader of the Mahdi Army, a Shia armed group that targeted US troops throughout the nine-year war in Iraq.