In its third apparent beheading of a Western hostage in less than a month, IS on Saturday released a video showing the execution of Briton David Haines and threatening the life of another British captive.
As President Barack Obama offered US support for its “ally in grief”, Cameron faced growing calls to allow Britain’s military to help in Washington’s planned assault against the jihadist group that has seized parts of Syria and Iraq.
France on Sunday condemned the “heinous murder” of Haines, urging international action against the jihadist group.
“The heinous killing of David Haines is yet another demonstration of why the international community must mobilise against (the Islamic State), a cowardly and abject organisation,” the French presidency said in a statement.
President Francois Hollande’s office also expressed France’s solidarity with Haines’ family and Britain.
US Secretary of State John Kerry was in Paris to push for a broad coalition against IS and Australian Prime Minister Tony Abbott committed 600 troops to the US-led effort, denouncing IS as a “death cult”.
Cameron will chair a meeting of the government’s Cobra emergency committee early Sunday in response to the online video, which featured a masked IS militant claiming the execution was in retribution for Britain’s role in the campaign against the group.
Cameron called the attack “a despicable and appalling murder of an innocent aid worker” and “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,” he said in a statement.
Two US journalists were murdered in similar circumstances in recent weeks.
Obama slammed the latest attack as “barbaric” and said the US “stands shoulder to shoulder tonight with our close friend and ally in grief and resolve”.
Britain has yet to join US air strikes against IS in Iraq, but has offered to arm Kurdish peshmerga fighters battling the militants in the north of the country, a move cited in the latest video as a reason for revenge.
Britain’s Foreign Office said it was “working as quickly as it could” to verify the two-minute-27-second clip, entitled “A Message to the Allies of America”.
The video opens with a clip of Cameron describing the British strategy of working with the Iraqi government to help arm Kurdish fighters against “these brutal extremist militants,” and to offer aid, diplomacy, and military help to pressure IS.
Haines then appears, dressed in an orange jumpsuit, and identifies himself before calmly explaining that he is paying the price for Cameron’s policy.
The attacker, who appears to be the same man as in the previous two beheading videos — tells Britain the alliance with the US will “accelerate your destruction” and will drag the British people into “another bloody and unwinnable war”.
At the end of the clip, he also threatens to execute another captive, identified in a caption by name as another British citizen.
Haines’s brother Mike paid tribute to a “good brother… who was recently murdered in cold blood.”
“He was, in the right mood, the life and soul of the party and on other times the most stubborn irritating pain in the ass,” he said in a statement.
“He was and is loved by all his family and will be missed terribly.”
Haines, 44, was taken hostage in Syria in March 2013 and was threatened in a video released this month showing the beheading by an IS militant of US journalist Steven Sotloff.
IS released a video claiming the execution of fellow US journalist James Foley on August 19.
Former head of the British army Richard Dannatt on Sunday piled pressure on Cameron to let the country’s military join a planned assault against IS, announced by Obama this week.
“What we absolutely need to do is not be cowed in any way by yet another foul murder of a hostage,” he told Sky News.
“We can support them (the US) to confront, attack and defeat the Islamic State jihadi fighters … and make sure this cancer is removed from the region before it spreads more widely.”
Obama on Wednesday set out a strategy that would include air strikes in Syria and expanded operations in Iraq, where US aircraft have carried out more than 160 strikes since early August.
He promised to build a wide-ranging international coalition against the jihadists and on Sunday Abbott said Australia would deploy 600 troops to the United Arab Emirates to join the US-led effort.
“There are obviously further decisions to be taken before Australian forces will be committed to combat operations in Iraq,” Abbott told a press conference in Darwin.
“Nevertheless, Australia is prepared to engage in international operations to disrupt and degrade ISIL (IS) because of the threat that this murderous death cult poses not just to the people of Iraq, not just to the people of the Middle East, but to the whole world including to Australia.”
France is to host an international conference on Iraq on Monday and President Francois Hollande’s office said the latest killing was further evidence of the need to take action against IS.
“The heinous killing of David Haines is yet another demonstration of why the international community must mobilise against (the Islamic State), a cowardly and abject organisation,” it said.