Britain’s Foreign Office said Sunday “all signs are that the video is genuine,” adding that it has no reason to believe otherwise. Cameron – who called the beheading an “act of pure evil” – has pledged to do “everything in our power to hunt down these murderers and ensure they face justice, however long it takes.”

The U.K. Parliament voted against military action in Syria last year, but some British lawmakers called for the U.K. to launch air strikes in response to Haines video. The video featured a black-clad militant clutching a knife and saying Haines’ execution was retaliation for the U.K.’s decision to arm Kurdish fighters battling the group in Iraq. President Barack Obama said the U.S. stands “shoulder-to-shoulder” with Britain in grief and resolve. France, which is holding an international conference on Monday about the threat of ISIS, also condemned Haines’ murder.