Update | British PM Cameron vows justice over Islamic State beheading of aid worker David Haines

British Prime Minister David Cameron vowed yesterday that Britain would do all it could to catch the killers of a British aid worker beheaded by Islamic State, which he condemned as an "act of pure evil".

In its third beheading of a Western hostage in less than a month, Islamic State released a video late on Saturday showing the execution of British aid worker David Haines and threatening the life of another British captive.

As US President Barack Obama offered US support for its "ally in grief", Cameron faced growing calls at home to commit Britain's military to Washington's planned assault against the jihadist group that has seized parts of Syria and Iraq.

US Secretary of State John Kerry was in Paris to push for a broad international coalition against Islamic State. He has secured the backing of 10 Arab states, including Saudi Arabia.

American efforts were bolstered yesterday by Australia's announcement that it was deploying 600 troops - including 200 special forces - to the region to join the effort against what Prime Minister Tony Abbott called a "murderous death cult".

France is hosting an international conference on Iraq today, and President Francois Hollande's office said the "heinous killing" of Haines was another reason why a global push against Islamic State was needed.

France believes that some 930 of its citizens or residents are involved in jihad in Iraq and Syria and estimates 36 have already died there.