US 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, US government sources said on Thursday.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 US 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 US belief that Iran was not involved, US 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 US 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 US troops in 2011.