An Iran-backed militia that killed four British hostages abducted in 2007 is suspected of being behind the kidnapping of three American citizens in Baghdad, according to Iraqi and U.S. intelligence sources.

Gunmen from the Shia militia, known as the League of the Righteous, are thought to have snatched the men from an apartment in the Doura neighbourhood in southeastern Baghdad on Sunday.

Iraqi and U.S. authorities have narrowed their investigations down to just two Shia militia groups, the League and the Saraya al-Salam, or Peace Brigades, loyal to the radical Shia cleric Moqtada al-Sadr.

Abduction: Three Americans working for a defence contractor in Baghdad, Iraq, were kidnapped from this apartment on Sunday by Iran-backed Shia militia known as the League of the Righteous, security sources said

Victim: The same militia group was responsible for the kidnapping of British computer expert Peter Moore (pictured) and his four bodyguards in May 2007 in Baghdad

The League was also responsible for the kidnapping of British computer expert Peter Moore and his four bodyguards in May 2007 in Baghdad.

Although Mr Moore was released in December 2009, all four of his bodyguards - Jason Creswell, Jason Swindlehurst, Alec MacLachlan and Alan McMenemy - were killed in captivity, in what became one of the worst kidnapping incidents in modern British history.

Officials added that while Iran is thought to support the organisation behind the kidnapping, which is also known by the Arabic name Asaib Ahl al-Haq, there is no reason to believe Tehran ordered the kidnapping.

The kidnapping of the U.S. security contractors could complicate relations between Washington and the Iraqi government, which relies on the Shia militias to lead the fight against ISIS.

A Western official revealed to news agency AP that the League was involved today.

‘Nobody can do anything in that neighborhood without the approval of those militias,’ an Iraqi police commander told the agency, referring to the area in which the contractors were abducted and supporting the official’s claims.

Nobody can do anything in that neighborhood [Doura] without the approval of those militias. Iraqi police commander

The League - one of Iraq's most powerful armed groups - claims to have launched thousands of attacks on both British and U.S. troops. It also claimed responsibility for the killing of five British soldiers when their helicopter was downed in Basra in May 2006.

An Iraqi intelligence source previously told Reuters: 'They were abducted because they are Americans, not for personal or financial reasons.'

The three men are employed by a small company that is doing work for General Dynamics Corp, one of the world's largest defense contractors.

The corporation is working in Iraq 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 Shia militias, many of which fought the U.S. and British military following the 2003 invasion and have previously been accused of killing and abducting American and British nationals.

Probe: The men were snatched from the Doura district (pictured) to put pressure on Iraqi Prime Minister Haider al-Abadi, sources said, though there is no indication that the operation was ordered by Tehran

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 U.S.

'The militias are resentful of the success of the army in Ramadi [a city in central Iraq] which was achieved with the support of the U.S.-led coalition and without their involvement,' he said.

Ramadi, the capital of Anbar province and formerly home to half a million people, was recaptured from majority-Sunni ISIS forces late last year.

Shia militias were kept out of the battle against ISIS 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 only: 'The picture is becoming a little bit more clear in terms of what might have happened.'

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.