The FBI said on Thursday it has identified the man behind the beheading videos of hostages James Foley Steven Sotloff and David Haines.

The name and nationality of the masked man, featured in the three videotaped executions, was not revealed by FBI Director James Comey, who spoke to journalists at the agency's headquarters in Washington DC.

The FBI chief also said that many Americans fighting with rebels in Syria have returned to the United States.

The string of videos began after the US launched air strikes in Iraq in July.

Islamic State has declared a caliphate across chunks of northern Iraq and Syria it has seized from tribes and authorities.

Each of the three videos opens with a direct address to a world leader