Cameron vows to hunt down 'IS' killers

Speaking after a meeting with intelligence and military officials on Saturday, Cameron said his government would "do everything we can to hunt down and find the people who are responsible for this."

"The murder of Alan Henning is absolutely abhorrent, it is senseless, it is completely unforgivable," he added. "The fact they could murder him in the way they did shows what we are dealing with."

A video released by the self-proclaimed "Islamic State" (IS) Friday night purports to show a masked militant beheading 47-year-old Alan Henning, who was abducted in December while transporting medical supplies to a hospital in northwest Syria.

Towards the end of the footage, fellow aid worker from the US, Peter Kassig, appears being threatened by the knife-wielding militant. The terror group is also believed to be holding several other Western hostages in Syria, including British journalist John Cantlie, who has appeared in three IS videos.

"We will use all the assets we have…to try and find these hostages...to defeat this organization which is utterly ruthless, senseless and barbaric in the way it treats people," Cameron said.

Henning, a father of two, was the fourth Western hostage to be beheaded by IS, following the release of similar videos showing the executions of David Haines, James Foley and Steven Sotloff. A fifth man, French national Herve Gourdel, was also beheaded, but by Algeria-based group Jund al-Khalifah which has links to IS.

Revenge for airstrikes

Friday's beheading video opens with a news piece annoucing the commencement of British airstrikes against IS in Iraq, which were approved in a parliamentary vote last week. British Royal Air Force (RAF) jets have since joined France and the US in conducting a number of strikes against militant targets in Iraq.

Following Henning's murder, US President Barack Obama warned the US would continue "standing together with our UK friends and allies, we will work to bring the perpetrators of Alan's murder - as well as the murders of Jim Foley, Steven Sotloff and David Haines - to justice," he said.

Meanwhile, Kurdish forces supported by US-led airstrikes managed to hold off IS militants around the Syrian border town of Kobani on Saturday. Dozens of IS militants were reported dead following the latest round of strikes, according to the group Syrian Observatory for Human Rights.

"The fighting is still ongoing at all fronts around Kobani and our fighters have scored major hits against Islamic State inflicting heavy casualties among their ranks," Redur Xelil, spokesman for the Kurdish People's Protection Units (YPG), told German news agency DPA.

The area has become the site of heavy fighting in recent weeks, as jihadists attempt to seize the town, sending thousands of refugees fleeing across the border into Turkey.

nm/kms (Reuters, AP, AFP, dpa)