'Pure Evil': ISIS Appears to Kill British Hostage in New Video

— -- The terror group ISIS appears to have executed a British hostage, as seen on a video released online today, making David Haines the third Western hostage the group is believed to have killed in such a public, brutal fashion.

Prior to his apparent beheading, Haines is seen kneeling in an orange jumpsuit next to an armed militant clad entirely in black. Haines gives a statement in which he says British Prime Minister David Cameron is "entirely responsible for [his] execution."

"You entered voluntarily into a coalition with the United States against the Islamic State [ISIS], just as your predecessor Tony Blair did, following a trend amongst British Prime Ministers who can't find the courage to say no to the Americans," Haines says.

In another section of the video, the would-be executioner says, "This British man has to pay the price for your promise, Cameron, to arm the peshmerga against the Islamic State." The peshmerga are Kurdish forces who, along with Iraqi forces and heavy American air support, dealt ISIS a major blow by wresting from the terror group control of the Mosul Dam last month.

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The new video is similar to two previously released by ISIS in which two American journalists, James Foley and Steven Sotloff, appeared to be separately beheaded. In those videos, a black clad militant addresses President Obama, warning America to stop its aggressive bombing campaign against ISIS.

Haines was previously seen at the end of the Sotloff video, ISIS implying that he would be next. Similarly, another hostage appears at the end of this new ISIS video.

Haines, 44, was abducted in Syria in 2013 as he worked for an international aid agency, according to The Associated Press.

Cameron reacted to the video on Twitter, saying the "murder of David Haines is an act of pure evil."

"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron's Twitter account said.

Haines’ brother, Michael, issued a statement about David who he said “was recently murdered in cold blood.”

“David was like so very many of us, just another bloke,” Michael Haines said in a statement issued by the British Foreign Office. “David was a good brother, there when I needed him and absent when I didn’t…He helped whoever needed help, regardless of race, creed or religion.

“David was most alive and enthusiastic in his humanitarian roles… He was and is loved by all his family and will be missed terribly,” Michael Haines said.

Earlier today, before the video emerged online, Haines' family released a public plea through the British government, urging his captors to contact them.

President Obama issued a statement saying the U.S. "strongly condemns the barbaric murder."

"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," Obama said.

In addition to the second hostage shown at the end of the video, ISIS is believed to be holding at least one other Westerner, a 26-year-old American woman.

Military special operations veterans told ABC News that in addition to the overt message delivered by Haines and his purported killer, the circumstances of the video’s production could be signal to Western intelligence. By shooting in broad daylight and in open desert, the officials said ISIS could be saying that it feels safe from the long reach of Western and allied militaries and intelligence.

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