UK 'will hunt down Haines killers'
Britain stands ready to take "whatever steps are necessary" to help an international push to destroy the "evil" extremist group who murdered a British aid worker, David Cameron said.
The Prime Minister hailed David Haines as a "British hero" and vowed to " hunt down those responsible and bring them to justice no matter how long it takes" after Islamic State (IS) posted a video of the beheading of the father of two.
A second British hostage, named as Alan Henning, was also shown - with a threat that he would be the next to be killed.
The killing appeared to have been carried out by the same British-accented jihadist responsible for the deaths of two American journalists held by the group - which is also known as Isil (Islamic State of Iraq and the Levant).
In an emotional statement after chairing a meeting of the Government's Cobra emergency committee to discuss the killing, the Prime Minister said the brutal crime would "strengthen our resolve" to take on the threat posed by the Islamic extremists at home and abroad.
British people would be "sickened" that a fellow citizen could have carried out such an act, the Prime Minister said and urged action to drain from society the "poison" of radicalisation - insisting IS and its adherents "are not Muslims, they are monsters".
He remained tight-lipped on Britain's role in the planned escalation of the military offensive in Iraq and Syria being co-ordinated by the US, and officials made clear there were no immediate plans to recall Parliament to discuss the use of British armed forces.
But while he ruled out sending ground troops, Mr Cameron left the door open to air strikes - a course being urged on him by former military chiefs and some MPs.
"The murder of David Haines at the hands of Isil will not leave Britain to shirk our responsibility with our allies to deal with the threat that this organisation poses. It must strengthen our resolve," he declared.
"Ultimately our security as a nation, the way we go about our everyday lives in this free and tolerant society that is Britain, has always depended on our readiness to act against those who stand for hatred and who stand for destruction. That is exactly what we will do."
Fears for Mr Haines's life had intensified when he was paraded as the next intended victim in the video of the death of journalist Steven Sotloff earlier this month and Mr Cameron paid tribute to the "extraordinary courage" shown by the Haines family since he was snatched in Syria in March last year.
News of Mr Haines's death came only hours after his family issued a plea to his captors to contact them.
"The whole country, like his grieving family, can be incredibly proud of what he did and what he stood for in his humanitarian mission," Mr Cameron said.
In a statement issued by the Foreign Office, Mike Haines said his brother, who previously served in the RAF and worked for the UN in the Balkans, was "just another bloke" who was "most alive and enthusiastic" in his humanitarian roles and will be "missed terribly".
His killing was met with horror by political and religious leaders as well as British Muslims and ACTED, the aid agency for which he was working when he was snatched in Syria in March last year, said it was "a crime against humanity".
US president Barack Obama said America - where the two previous killed hostages, both journalists, were from - "stands shoulder to shoulder with our close friend and ally in grief and resolve".
Deputy Prime Minister Nick Clegg said the murder was "barbaric" and promised the Government "will not rest until these killers face justice" and Labour leader Ed Miliband said he was "sickened" at the murder.
In a sermon at Bristol Cathedral , the Archbishop of Canterbury said the aid worker was "evilly killed in the place he was serving in love for its suffering people".
Dr Shuja Shafi, secretary general of the Muslim Council of Britain, said there was "nothing in our faith that condones such behaviour.
"Muslims in Britain and around the world have condemned these people, and the arguments they use have been refuted comprehensively as being far from the religion of Islam."
Among those calling for the UK to join air strikes was former head of the Army Lord Dannatt.
"We have got to do the right thing even if it's not currently the popular thing or otherwise we will regret not taking decisive action," he told Sky News, w elcoming Australia's pledge to contribute 600 troops and up to 10 military aircraft to the campaign.
Former naval chief Admiral Lord West of Spithead said: "We have a perfect right to use all means at our disposal to do something about them," he said.
Scotland's First Minister Alex Salmond convened a "resilience meeting" to discuss the implications of the "act of unspeakable barbarism" for the Scottish Government, including the need to ensure there was no backlash against the Muslim community.
He criticised the lack of a clear wider international strategy to tackle the threat of IS and insisted any action should only be taken with the approval of the United Nations.
Mr Cameron said the UK would seek to "mobilise the broadest possible support to bear down" on IS at the United Nations as efforts continue to form a coalition of countries - especially those in the region - to take part in a concerted offensive.
"This is not about British combat troops on the ground, it is about working with others to extinguish this terrorist threat. A s this strategy intensifies we are ready to take whatever steps are necessary to deal with this threat and keep our country safe," he said.
The head of international police agency Interpol said those responsible would "one day face justice" as he pressed for greater co-operation to root out foreign fighters in Syria and Iraq.
Speaking during a visit to Qatar to discuss security in the region, s ecretary general Ronald K Noble said: "The cowardly murder of an innocent man whose humanitarian vocation was to improve the lives of others in the very region he met his brutal death is testimony to the twisted and depraved nature of the so-called Islamic State.
"On behalf of the international police community Interpol strongly condemns the murder of David Haines and deplores the deaths of all other innocents in the region at the hands of Islamic State militants.
"However long it takes, they will one day face justice for their barbaric and senseless actions.
"Interpol continues to facilitate exchanges between its 190 member countries on information relating to suspected terrorists and foreign fighters.
"But this must be complemented by efforts on the ground, including through the provision of global police tools and databases to officers on the front line."
Mr Salmond said: "This was an act of unspeakable barbarism.
"We offer our sincerest condolences to the Haines family and friends who have borne these absolutely terrible circumstances with great fortitude in recent months. Supporting the family and ensuring they get the privacy they have asked for is of primary importance to us at this time.
"Earlier today I convened a meeting of the Scottish Government's Cabinet Sub-Committee for Resilience (CSC-SGOR) to look at our specific responsibilities in these matters and we will continue to work closely with Police Scotland, the Crown Office & Procurator Fiscal Service and the Foreign and Commonwealth Office in the coming days.
"We will also work with the communities of Scotland, which is of great priority. Scotland's Muslim community are a strong and valued part of Scottish society, and they should in no way be held responsible for the extremism of Isil. But we must have total vigilance, and be clear that any aggravated racial or religious attack will be met with the full force of the law and will not be tolerated."