The three U.S. nationals who went missing in Baghdad were reportedly kidnapped and are being held by an Iranian-backed Shi'ite militia, Iraqi intelligence and U.S. government sources said Tuesday, noting that they believe Iran wasn't involved in the kidnapping, nor do they believe they are being held there.

"They were abducted because they are Americans, not for personal or financial reasons," one of the Iraqi intelligence sources in Baghdad said, according to Reuters.

News of the kidnapping broke Sunday after the Arab news channel, al-Arabiya, citing its own sources, reported that three Americans had been abducted by militias in Baghdad as they were on their way to Baghdad International Airport.

The next day, a security official clarified the situation, saying that the three were kidnapped from a "suspicious apartment," using language that implied the apartment, was in fact, a brothel, according to AFP.

The kidnapping marked just the latest in a string of foreigner kidnappings in Iraq in recent years. However, victims of such abductions are usually Qataris and Turks, with Iraqis suffering the most from kidnappers seeking ransoms or to settle scores. It had been years since a U.S. citizen has been abducted in Iraq.

Iraqi parliament speaker Salim al-Juburi condemned the kidnapping, saying it would harm the country's relations with other states. 

"The kidnapping of the Americans citizens, and before them the Qatari hunters, whose fates are still unknown, undoubtedly indicate the growing activities of the organized gangs in Iraq," he said, according to The New York Times. "This will affect the relations of Iraq with its neighbors, and friendly and allied countries." 

In the meantime, the State Department said it was working with Iraqi authorities to locate the missing Americans.

"We are working with the full cooperation of the Iraqi authorities to locate and recover the individuals," the statement said.