LONDON — Condemning an act of "pure evil", Prime Minister David Cameron vowed Sunday that Britain would do all it can to catch the killers of a British aid worker beheaded by the Islamic State.



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 extremist group that has seized parts of Syria and Iraq.



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". — AFP