The United States vowed justice for the beheading of a second American journalist Steven Sotloff by Islamic State militants, depicting in a video released yesterday.

The video, entitled 'A second message to America', was deemed authentic by US and UK intelligence services.

"Whatever these murderers think they will achieve by killing innocent Americans like Steven, they have already failed," US President Barack Obama said. (See video here)

"They failed because like people around the world, Americans are repulsed by their barbarism, we will not be intimidated, their horrific acts only unite us, and stiffen our resolve to take this fight against these terrorists," he added.

"Those who make the mistake of harming Americans will learn that we will not forget and our reach is long and justice will be served."

The video, featuring an IS militant believed to be a British citizen who was shown in the first video which depicted the beheading of the US journalist James Foley, includes a warning to the UK government that the execution of a British hostage would soon follow.

The UK, which has till now fallen short of joining the US in carrying out air strikes over Iraq in an effort to aid Kurdish forces, held an emergency Cobra meeting today.

The British Prime Minister David Cameron told MPs that the beheadings were  "utterly abhorrent and barbaric" but warned against "knee jerk reactions".

"We are doing everything we can to reassure the family of the British hostage who was shown in the video," UK Foreign Secretary said, adding that air strikes had not been ruled out.

"If we judge that airstrikes could be beneficial ... then we will certainly consider them. But we have made no decision to do so at the moment."