Islamic State (Isis) released a video Sunday allegedly showing the beheading of a dozen Syrian prisoners and a US aid worker.

The fanatical terrorist network, which has taken advantage of the West's backing for radical Islamists trying to overthrow the Bashar al-Assad government to seize control of about a third of Syria and much of neighbouring Iraq, uses propaganda beheading videos to intimidate its opponents.

The video showed people identified as Syrian military officers and pilots dressed in blue jumpsuits having their heads cut off, alongside a man it said was US citizen Peter Kassig.

The United States said it was working to determine whether the video was authentic or not.

Mr Kassig's family said they would prefer "our son is written about and remembered for his important work, not in the manner the hostage takers would use to further their cause."

A Syrian who worked with Mr Kassig said: "Those who killed him claimed to have done it in the name of Islam. I am a Muslim. His killers are not."

The film says it was taken in northern Syrian town Dabiq, which Isis claims will be the scene of an apocalyptic battle against unbelievers based on a 1,300-year-old tale it regards as a prophecy.

It includes a warning from a masked terrorist with a British accent who threatens a similar fate for US troops.

The audio of the video had been distorted to make identification of the killer who speaks more difficult.