BAGHDAD/WASHINGTON - 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.Unknown gunmen seized the three on Friday from a private residence in the southeastern Dora district of Baghdad, Iraqi officials say. They are the first Americans to be abducted in Iraq since the withdrawal of U.S. troops in 2011.The U.S. sources said Washington had no reason to believe Tehran was involved in the kidnapping and did not believe the trio were being held in Iran, which borders Iraq."They were abducted because they are Americans, not for personal or financial reasons," one of the Iraqi sources in Baghdad said.The three men are employed by a small company that is doing work for General Dynamics Corp, under a larger contract with the U.S. Army, according to a source familiar with the matter.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.Baghdad-based analyst Hisham al-Hashemi, who advises the government, said the kidnappings were meant to embarrass and weaken Iraqi Prime Minister Haider al-Abadi, who is trying to balance his country's relations with rival powers Iran and the United States."The militias are resentful of the success of the army in Ramadi which was achieved with the support of the U.S.-led coalition and without their involvement," he said.SECTARIAN TENSIONSShi'ite militias were kept out of the battle against Islamic State in Ramadi for fear of aggravating sectarian tensions among the Sunni population in the western city.Baghdad touted the military's advance there last month, with backing from coalition airstrikes, as evidence of a resurgent army after it collapsed in 2014.The State Department said on Sunday it was working with Iraqi authorities to locate Americans reported missing, without confirming they had been kidnapped.Asked about the kidnapping at the daily U.S. State Department news briefing on Tuesday, spokesman John Kirby said: "The picture is becoming a little bit more clear in terms of what might have happened." He provided no details.Kirby declined to say whether Secretary of State John Kerry had contacted Iranian Foreign Minister Javad Zarif about the kidnapping.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.However, the United States imposed sanctions on 11 companies and individuals on Sunday for supplying Iran's ballistic missile program.