On Saturday, the Takfiri group posted the footage consistent with the earlier ones showing the execution of two American journalists James Foley and Steven Sotloff.

A masked militant appeared in the video, entitled, "A Message to the Allies of America," saying the execution was in response to UK Prime Minister David Cameron’s engagement in a US-led coalition against the militants.

The British prime minister had earlier talked of efforts “to help make sure the Kurds have the arms they need to fight off” the terrorist group.

In the footage, Haines also accused British premiers for not having the “courage to say no to the Americans.”

“Unfortunately, it is we, the British public that will, in the end, pay the price for our Parliament's selfish decisions," Haines said, referring to Britain’s engagement in war on the ISIL.

The British victim, who is a 44-year-old father of two from Perth in Scotland, was reportedly captured in Iraq last year.

Another hostage, identified as Alan Henning, was shown at the end of the video, with the masked terrorist saying he would be killed if Cameron continued fighting the ISIL.

The video was shot in a desert setting similar to the two previous ones with the victim wearing an orange jumpsuit.

US President Barack Obama also said that Washington would stand “shoulder to shoulder,” with the UK following "the barbaric murder" of Haines.

"We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice," Obama said.

Iraq has been fighting the ISIL terrorists since they took control of Mosul on June 10. The takeover was followed by the fall of the city of Tikrit, located 140 kilometers (87 miles) northwest of the capital Baghdad. The control of Tikrit was later retaken by the Iraqi army.

The Takfiri terrorists currently control parts of eastern Syria and Iraq’s northern and western regions. They have threatened all communities, including Shias, Sunnis, Kurds, Christians, Izadi Kurds and others, as they continue their atrocities in Iraq.

Senior Iraqi officials have blamed Saudi Arabia, Qatar and some Persian Gulf Arab states for the growing terrorism in their country.

The terrorist group has links with Saudi intelligence and is believed to be indirectly supported by the Israeli regime