The Islamic State released a third execution video today showing the beheading of a British aid worker, David Haines, for the first time targeting the United Kingdom.

Prime Minister David Cameron tweeted in response, "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

This is the third execution by the Islamic State, formerly ISIS, which is sending messages to the Western powers to back off their attacks on the caliphate that includes portions of Iraq and Syria, the Daily Mail reported.

"David Cameron, I hold you entirely responsible for my execution," Haines said in the video.

On Saturday, the family released a plea to Haines' captors through the British Foreign Office asking for the captors to make contact with them, CNN reported.

Haines was a logistics and security manager for the Paris-based Agency for Technical Cooperation and Development, and the 44-year-old was abducted in March 2013 near a refugee camp in Atmeh, Syria, where he was working to arrange delivery of humanitarian aid.

In previous videos showing the killings of American freelance journalists Steven Sotloff and James Foley, ISIS spoke to President Barack Obama warning that there would be more killings if the airstrikes did not stop.

Obama recently declared a strong defense strategy including funding military of the affected countries but refrained from an on-the-ground war by U.S. troops.

The video, titled "A Message to the Allies of America," starts with a video clip of Cameron speaking about supporting the Kurds in their fight against the Islamic State, Daily Mail reported.

Similar to the previous two videos, it then cuts to the now all-too-familiar barren landscape with a militant dressed in all black and a hostage dressed in orange kneeling next to him. However, instead of a sandy area, they appear to be on a rocky surface.

The militant speaks to Cameron, warning that his alliance with the U.S. to confront ISIS will only drag the British people into "another bloody and unwinnable war."

The militant reappears at the end next to another British individual identified as Alan Henning and threatens that if Cameron persists in fighting the Islamic State, he will join Obama in having the blood of his people on his hands.

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