WASHINGTON/DAVOS (Reuters) - U.S. intelligence agencies investigating the kidnapping of three Americans in Baghdad last week are focusing their probe on three militant Islamic groups closely affiliated with Iran, U.S. government sources said on Thursday.

Asaib Ahl al-Haq Shi'ite militia fighters from the south of Iraq run during a mission to take control of Sulaiman Pek village from Islamist State militants, in the northwest of Tikrit city September 1, 2014. REUTERS/Youssef Boudlal

Asaib Ahl al-Haq, Kata’ib Hezbollah and the Badr Organization are the principle focus of the probe into how the men were snatched in the Dora neighborhood, south of Baghdad, the sources said.

The U.S. government does not know if any of the three groups seized the men. While the groups have close ties to Iran, sources said the United States does not believe Iran had a hand in the kidnapping nor that the three are being held in Iran.

Despite the U.S. belief that Iran was not involved, U.S. Secretary of State John Kerry on Thursday said he had asked Iran for help in finding them. Iraq’s prime minister, speaking in Davos, Switzerland, said he doubted Iran was involved.

Kerry told reporters that he and Iranian Foreign Minister Mohammad Javad Zarif discussed the case of the three men during a meeting on Wednesday. Iraqi intelligence and U.S. government sources on Tuesday said the men were kidnapped and were being held by an Iranian-backed Shi’ite militia.

They are the first Americans to be abducted in Iraq since the withdrawal of U.S. troops in 2011.

“I asked him (Zarif) for whatever help, if Iran knew any way to provide help, or if there was some way they could have impact in getting the right outcome,” Kerry told reporters.

“He said he would take it under advisement and try to do what they can. He didn’t have any immediate knowledge whatsoever about it,” Kerry added.

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.

‘THEY JUST WENT MISSING’

Asaib Ahl al-Haq, Kata’ib Hezbollah and the Badr Organization are Shi’ite militia groups that are part of Iraq’s Popular Mobilization Front, a group closely tied to Iran, according to the New York-based Counter Extremism Project advocacy group.

The Iraqi government has struggled to rein in the Shi’ite militias, many of which fought the U.S. military following the 2003 invasion. Shi’ite militias have previously been accused of killing and abducting American nationals.

Speaking before a meeting with Kerry on Thursday, Iraqi Prime Minister Haider al-Abadi was asked by a reporter if he thought there was an Iranian link to the mens’ disappearance.

“I don’t know about that. I doubt it very much. We don’t know if they have been kidnapped...they just went missing,” Abadi said in response to the question.

Unknown gunmen seized the three on Friday from a private residence in the southeastern Dora district of Baghdad, Iraqi officials say.

Kerry said the United States was working closely with Iraq on the issue.

“They are really investigating this. He (Abadi) is looking at it. He was not able to shed light on the who, where or what and they are still trying to get all of that piece together.”

Some analysts believe the kidnappings were meant to embarrass and weaken Abadi, who is trying to balance Iraq’s relations with rival powers Iran and the United States.

Hostility between Tehran and Washington has eased 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.