The Prime Minister David Cameron today gave his strongest indication yet that he will give the go-ahead to strikes against ISIL in Iraq and Syria as he pledged to hunt down the "monsters" who murdered British hostage David Haines.

Speaking after an emergency Cobra meeting, Cameron described Haines as a "British hero", said the UK is "incredibly proud" of him and said that he has an "iron determination" to destroy Isil.

He said: "David has been murdered in the most callous and brutal way imaginable by an organisation which is the embodiment of evil.

"We will hunt down those responsible and bring them to justice no matter how long it takes.

"David Haines was an aid worker. He went into harms way not to harm people but to help his fellow human beings in the hour of their direst need – from the Balkans to the Middle East.

"His burning desire to help others has cost him his life. The whole country, like his grieving family, can be incredibly proud of what he did and what he stood for in his humanitarian mission. David Haines was a British hero."

He described Isil as an "evil" and "callous" organisation.

"They are killing and slaughtering thousands of people – Muslims, Christians, minorities – across Iraq and Syria.

"They boast of their brutality; they claim to do this in the name of Islam. That is nonsense. Islam is a religion of peace. They are not Muslims, they are monsters."

The emergency talks came after the beheading of Haines by Islamic State militants, propelling the possibility of military intervention to the top of the agenda.

Senior public servants from the Armed Forces, security services, Foreign Office and Home Office met in Whitehall to discuss how best to respond to the growing crisis.

Although Parliament will not be recalled to discuss the beheading of Haines, described by Cameron as an "act of pure evil", both Mr Cameron and US president Barack Obama vowed to bring the Islamists responsible to justice. It is believed the murderers of Haines could include, as with the two US hostage beheadings, a British Muslim who went to the Middle East to wage "jihad" against the West.

The Archbishop of Canterbury Justin Welby tweeted: "In every church let us pray for the family of David Haines, evilly killed in the place he was serving in love for its suffering people."

Cameron arrived at Downing Street early this morning after a video appeared online at midnight that showed the murder of Haines, aged 44, captured in Syria last year. The footage includes a threat to kill a second British hostage.

The video opens with a clip Cameron and shows Mr Haines dressed in orange overalls, kneeling before a man holding a knife in a desert-style area.

Haines is recorded saying to camera: "I would like to declare that I hold you David Cameron entirely responsible for my execution.

"You entered voluntarily into a coalition with the United States against the Islamic State just as your predecessor Tony Blair did, following a trend against our British prime ministers who can't find the courage to say no to the Americans.

"Unfortunately it is we the British public that in the end will pay the price for our Parliament's selfish decisions."

The murderer, speaking in the British accent similar to the last two beheading videos, says: "This British man has to the pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State.

"Your evil alliance with America which continues to strike the Muslims of Iraq and most recently bombed the Haditha Dam will only accelerate your destruction and playing the role of the obedient lapdog Cameron, will only drag you and your people into another bloody and unwinnable war."

The murderer, dressed in black, then saws Haines' throat with a knife. The dead body is then shown.

The murderer says: "If you, Cameron, persist in fighting the Islamic State then you like your master Obama will have the blood of your people on your hands."

Mr Haines, who had a teenage daughter in Scotland from a previous marriage and a four-year-old daughter, Athea, in Croatia with his present wife, worked with the Royal Mail after leaving school then joined the RAF as an aircraft engineer. He served with the UN in the Balkans.

After leaving the RAF he worked for Scotrail.

The video led to wide international outrage.

Cameron said: "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

Obama, who recently announced that the US is to take action against Islamic State in Syria, said: "We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world."

Deputy Prime Minister Nick Clegg said: "My thoughts at this time are with the family of David Haines who have experienced something no family should ever have to go through.

"This murderous organisation calls itself Islamic State.

"But it is not a state, it is a brutal terrorist outfit whose actions are an affront to every peace-loving Muslim around the world.

"No religion could possibly justify such grotesque acts."

The victim's brother Mike Haines: "His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair.

"David was a good brother. He was, in the right mood, the life and soul of the party and on other times the most stubborn irritating pain in the ass."

ACTED, the aid agency David Haines was working for when captured, said in a statement: "ACTED aid agency is deeply appalled and horrified by the assassination of David Haines. ACTED strongly condemns with the utmost of force these crimes. In this tragic moment, our thoughts are with his family, friends and loved ones. All of the ACTED team empathise and share their pain.

"David was a new member of the ACTED team supporting the emergency humanitarian response for the displaced Syrian people in Atmeh camp near to the Turkish border.

"David was appreciated by the ACTED team and all those around him, notably for his generosity, commitment, and his professionalism. Since his abduction, David has constantly been at the forefront of our minds and efforts. David will remain in the hearts and minds of everyone in ACTED.

"The horrible assassination of David, an aid worker, goes against all humanitarian principles and is a crime against humanity. This barbaric crime must not remain unpunished. "

Lord Dannatt, former head of the British Army and a committed Christian, said the hostage killings - and the threat of more - must not stop the UK joining international action against IS "in the strongest possible way" including air strikes.

General Dannatt, currently Constable of the Tower of London, added: "Even these serial repetitions of murders being put on television screens, they must not lead our governments to conclude that this is too difficult, too dangerous and we do nothing."

Admiral Lord West, former First Sea Lord, said: "When a group of people - they like to call themselves a state but they're not, they're just a group of thugs - start killing people. We have a perfect right to do something about it."

Colonel Richard Kemp, the former commander of British forces in Afghanistan, said: "The problem is, if we allow things like this to happen to our citizens, we're showing weakness and encouraging other jihadists to do the same thing."

Archbishop of Canterbury Justin Welby tweeted: "In every church let us pray for the family of David Haines, evilly killed in the place he was serving in love for its suffering people."