The three American nationals who were reported missing in Baghdad since last week are believed to have been abducted by Shia militias backed by Iran, Iraqi and US officials have reportedly said. 

The three Americans, who were contractors for the US Army, had been kidnapped by gunmen in the Dora district of Baghdad on Friday. 

"They were abducted because they are Americans, not for personal or financial reasons," an Iraqi official told Reuters

The US government sources told the agency that they did not believe the men were being held in Iran. 

While the Sunni militant group Islamic State is largely known to be behind kidnappings and killings in Iraq in recent years, Shia militia are also known to have carried out such acts against Americans in the country. 

According to Kurdish news site Rudaw, Sunni parliamentarians had on Tuesday urged Iraqi Prime Minister Haider al-Abadi to disband Shiite militias who have been involved in deadly sectarian violence. 

US State department spokesman John Kirby had said in a statement on Sunday that the US was working with Iraqi authorities to locate the missing Americans, the first to be kidnapped since the 2011 withdrawal of US troops from Iraq. 

The local police had set up checkpoints around the neighbourhood from where the three people had gone missing.