Updated 7.10am Saturday - UK Prime Minister David Cameron has vowed to "hunt down" the terrorists responsible for the beheading of British aid worker Alan Henning, as the killing prompted horror and condemnation around the world.

A video showing the brutal murder of the 47-year-old former taxi driver from Salford - who was kidnapped last December in Syria by IS militants - was posted on the internet by the group late yesterday.

Henning, known to friends as the 'taxi driver with the golden heart,' made several trips to Syria to deliver aid. He was the only Christian in an aid convoy which entered Syria last Christmas and he was captured the following day. All his colleagues were released.

The Prime Minister said the killing "shows just how barbaric and repulsive these terrorists are", and promised to "do all we can to hunt down these murderers and bring them to justice".

French president Francois Hollande spoke of his "outrage" at the killing, US president Barack Obama condemned the "brutal murder", and the United Nations Security Council described it as "heinous and cowardly".

The footage shows Mr Henning kneeling beside a knifeman dressed in black in a desert setting.

It is the second such murder of a UK hostage after fellow aid worker David Haines was killed last month.

Two American journalists, James Foley and Steven Sotloff, were also beheaded by the terrorist organisation, which is also known as Isil.

The Prime Minister, who last night arrived back in the UK after visiting British troops including those engaged in air strikes on IS in Iraq, said: "The brutal murder of Alan Henning by Isil shows just how barbaric and repulsive these terrorists are.

"My thoughts and prayers (tonight) are with Alan's wife Barbara, their children and all those who loved him.

"Alan had gone to Syria to help get aid to people of all faiths in their hour of need. The fact that he was taken hostage when trying to help others and now murdered demonstrates that there are no limits to the depravity of these Isil terrorists.

"We will do all we can to hunt down these murderers and bring them to justice."
The gruesome video appears to show Mr Henning, who is dressed in orange, say: "I am Alan Henning. Because of our Parliament's decision to attack the Islamic State, I, as a member of the British public, will now pay the price for that decision."

The killer, who speaks with a British accent and is believed to be the man responsible for the previous beheadings, then makes a direct statement to the Prime Minister: "The blood of David Haines was on your hands, Cameron. Alan Henning will also be slaughtered, but his blood is on the hands of the British Parliament."

The video then apparently shows the jihadist alongside an American hostage identified as Peter Kassig.

Addressing the US president, the militant says: "Obama, you have started your aerial bombardment in Sham (Syria), which keeps on striking our people. So it is only right that we continue to strike the necks of your people."

Washington said the killing of Mr Henning "is again yet another just very clear example of the brutality of this group, and why the president has articulated and is moving out in a comprehensive way to degrade and destroy Isil".

Lisa Monaco, assistant to the president for homeland security and counter-terrorism, said: "Our hearts go out to the British aid worker who we believe is in that video and to the remaining hostages and to their families."

The Foreign Office said it was offering the Henning family "every support possible".

The latest video emerged a week after the House of Commons overwhelmingly backed the deployment of British military forces to bomb IS positions in Iraq.
With Labour support, MPs voted by a majority of 481 to endorse joining the US-led coalition confronting the extremists - although extending the mission into Syria is yet to be debated.

RAF jets began bombing missions at the weekend and Mr Cameron earlier announced the deployment of two more Tornado fighter bombers as he visited the Cyprus base from where they are operating.

The two additional GR4s lift the number to eight engaged in raids to support local ground forces, especially Kurdish Peshmerga forces.
Mr Henning was last seen at the end of a video released last month following the beheading of fellow British captive Mr Haines.

His wife Barbara earlier this week pleaded for him to be freed.

"I ask Islamic State: please release him, we need him back home," she said.
"Some say wrong time, wrong place. Alan was volunteering with his Muslim friends to help the people of Syria. He was in the right place doing the right thing."

Mrs Henning revealed she had received an audio message of her husband pleading for his life, and had been told that a Sharia court had found him innocent of being a spy.

Deputy Prime Minister Nick Clegg offered his "sincere condolences" to Mr Henning's family, saying: "We are resolved to defeat this evil."

Labour leader Ed Miliband called the murder "appalling and barbaric", adding: "We will do everything we can to support the efforts of the Government to bring those guilty of this terrible act to justice."

The release of the footage came just hours after the father of British journalist John Cantlie, who is being held by IS, appealed for his son's release.
Paul Cantlie said in a video from his hospital bed: "To those holding John: please know that he is a good man.

"He sought only to help the Syrian people and I ask you from all that is sacred to help us to allow him to return safely to those he loves and those who love him."

In a statement, Mr Kassig's parents Ed and Paula said: "The Kassig family extends our concern for the family of Alan Henning. We have read about his work and his generous character with great respect and admiration.

"We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe."

The United Nations Security Council said: "This crime is a tragic reminder of the increasing dangers volunteer humanitarian personnel face every day in Syria.

"It also once again demonstrates the brutality of ISIL, which is responsible for thousands of abuses against the Syrian and Iraqi people."