The United States condemned the killing of an American journalist, James Foley, by Islamic State militants, after a video was released showing Foley being beheaded.

"The United States of America will continue to do what we must do to protect our people. We will be vigilant and we will be relentless. When people harm Americans anywhere, we do what’s necessary to see that justice is done and we act against ISIL, standing alongside others," President Barack Obama said at a press conference on Wedcnesday.

"The people of Iraq, who with our support are taking the fight to ISIL must continue coming together to expel these terrorists from their community," he added.

The video which was sent as "A message to the US", included an audio message by an ISIL militant, speaking in English with a distinctly British accent, which warned the US that more Americans would be killed if air strikes in Northern Iraq did not cease.

Condemning the killing as "barbaric and brutal", the British Prime Minister David Cameron told the BBC it was "deeply shocking" that a British citizen may be responsible.

"We know that far too many British citizens have travelled to Iraq and travelled to Syria, to take part in extremism and violence, and what we must do is to redouble our efforts to stop people going," he said.

"[To] take away the passports of those contemplating travel, to arrest and prosecute those that take part in this extremism and violence, to take this extremist material off the internet, and do everything we can to keep our people safe."

The UK's Metropolitan police warned that "viewing, downloading or disseminating" the video message by ISIL may be prosecuted under anti-terror legislation.