The U.S. sources said Washington had no reason to believe Tehran was involved in the kidnapping.
Three U.S. citizens who disappeared last week in Baghdad were kidnapped and are being held by an Iranian-backed Shi'ite militia, two Iraqi intelligence and two U.S. government sources said on Tuesday.
The U.S. sources said Washington had no reason to believe Tehran was involved in the kidnapping and does not believe the trio are being held in Iran, which borders Iraq.
"They were abducted because they are Americans, not for personal or financial reasons," one of the Iraqi intelligence sources in Baghdad said.
Unknown gunmen seized the Americans on Friday from a private residence in the capital's southeastern Dora district, according to Iraqi officials.
The Iraqi government has struggled to rein in the Shi'ite militias, many of which fought the U.S. military following the 2003 invasion and have previously been accused of killing and abducting American nationals.
No U.S. citizens have been abducted in Iraq since U.S. troops withdrew from the country in 2011.
The State Department said on Sunday it was working with Iraqi authorities to locate Americans reported missing, without confirming they had been kidnapped.
Hostility between Tehran and Washington has eased in recent months with the lifting of crippling economic sanctions against Iran in return for compliance with a deal to curb its nuclear ambitions and a recent prisoner swap.