WASHINGTON (Reuters) - The United States Justice Department is conducting a criminal investigation into the death of American journalist James Foley, Attorney-General Eric Holder said on Thursday.

Mr Foley was beheaded by the Islamist militant group the Islamic State of Iraq and Syria, an act shown in a video released on Tuesday in which the group called for the United States to end its airstrikes in Iraq.

US President Barack Obama responded that the United States would be relentless in fighting the organisation despite the killing.

The identity of Mr Foley's killer, whose face was covered in the video, is unknown.