David Haines Beheading Video
A video purportedly showing threats being made to a man Islamic State (IS) named as David Haines by a masked IS fighter in an unknown location in this still image from video released by Islamic State September 2, 2014. REUTERS/Islamic State via Reuters TV

International support for the US-initiated air strikes in Iraq and Syria to wipe out the extremist militant group Islamic State (IS) continues to gain traction, with British Prime Minister David Cameron expected to announce an air strike next week. He gave a hint of Britain's military plan in response to the Saturday beheading of British aid worker David Haines.

Read: Islamic State: Extremist Group Beheads Brit Aid Worker David Haines; Another UK Citizen Alan Henning Next In Line

Following the decapitation, the third victim of IS after American journalists James Foley and Steven Sotloff, Mr Cameron described the extremist group as "the embodiment of evil." He warned that the jihadists who have taken the lives of Sotloff and Foley and now Haines would be hunted down and brought to justice.

"We cannot just walk on by if we are to keep this country safe. We have to confront this menace," the British PM vowed.

He will make the pitch against the IS after the Thursday referendum on the proposed Scottish independence, specifically outline the plan at the UN General Assembly to battle the IS. Britain's more hardline policy contrasts with a statement issued by the country foreign secretary that Britain would not bomb the group's targets in Syria but the PM has not ruled out anything.

The initial reluctance of the Liberal Democrats and Labour to endorse air strikes in Syria is because military action could have legal issues raised if done without the government consent. Previously, Britain has promised to arm Kurdish forces and provide humanitarian help.

Haines, 44, was raised in Scotland and had two daughters who lives in Croatia with his wife Dragana. He was in Syria just for three days, working to establish refugee camps, when he was kidnapped by IS rebels in March 2013.

Following Haines's decapitation, an international conference with over 20 nation-participants opened on Monday in Paris to discuss the threat from IS, which now controls one-third of Iraq. The gathering is hosted by French President Francois Hollande and Iraqi President Fuad Masum.

According to the US, there are around 40 countries that have agreed to be part of the battle against the IS, including Australia which promised on Sunday to send 10 military jets and 600 personnel.

YouTube/TomoNews US