U.K. Prime Minister David Cameron condemned Saturday the apparent beheading of British aid worker David Haines by the Islamic State, formerly known as ISIS. The militant group released a video it claimed showed the execution of the 44-year-old father of two.

“The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude,” Cameron wrote on his official Twitter account. “We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes.”

The British Foreign Office said it was “working urgently to verify” the authenticity of the purported Haines beheading video. If authentic, the government organization said that the act would constitute “another disgusting murder,” Sky News reports.

ISIS’ release of the purported Haines execution video was confirmed by SITE Intel Group, the same organization that first obtained footage of the Islamic State’s beheading of American journalist Steven Sotloff. Stills from the video depicted a masked man who claimed Haines was killed because Cameron and the British government promised to arm Kurdish fighters to combat the Islamic State in Iraq.

“This British man has to pay the price for your promise, Cameron, to arm the peshmerga against the Islamic State,” the masked man reportedly said, addressing U.K. Prime Minister David Cameron. The militant also threatened to execute a second British man if the U.K. continued military action against the Islamic State.

A jumpsuit-clad Haines spoke in the video and said that Cameron was directly responsible for his execution, Reuters reported. “You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor, Tony Blair," he said. “Following a trend amongst our British Prime Ministers who can’t find 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 decision.”

White House National Security Council spokeswoman Caitlin Hayden declined to comment on the video. She referred all further inquiries to the British government.

Haines was kidnapped in Syria while working with a French organization that assists refugees from the Syrian Civil War. He was taken captive in March 2013.