In an emotional statement at 10 Downing Street, British Prime Minister David Cameron said Britain will hunt down those responsible for the death of British aid worker David Haines.
Speaking after an emergency committee meeting of top military and political officials, Cameron paid tribute to the 44-year-old father of two, who was held hostage and killed by Islamic State militants.
"David Haines was a British hero. The fact that an aid worker was taken, held and brutally murdered at the hands of ISIL sums up what this organization stands for,” Cameron said.
'They are monsters'
Cameron said the Islamic State group is a fanatical organization that is planning attacks across Europe. He said it claims to act in the name of Islam, but he stressed that Islamic State militants "are not Muslims, they are monsters."
Image taken from video released by Islamic State militants, identified by private terrorism monitor SITE Intelligence Group, purportedly shows masked militant in desert landscape threatening to kill British David Cawthorne Haines, Sep. 2, 2014.
Cameron said Britain must confront the menace of Islamic State and will do so by working with the Iraqi and Kurdish regional government, continuing to provide humanitarian aid to those who have fled the group, and continuing to support U.S. military actions against Islamic State militants. He said the group will be defeated through a comprehensive and sustained counter-terrorism strategy.
Haines was seized in Syria in 2013 and held by Islamic State militants, who have already decapitated two U.S. captives, as well as Lebanese and Kurdish opponents.
A video of the aid worker's beheading was released late Saturday. The video begins with a clip of Cameron and then features a man who appears to be Haines dressed in orange overalls, kneeling in front of a masked man holding a knife.
The victim says: "My name is David Cawthorne Haines. I would like to declare that I hold you, David Cameron, entirely responsible for my execution."
The militant, who appears to have a British accent, then says: "This British man has to pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State."
The executioner warned that the decision will drag the British people into "another bloody and unwinnable war."
The U.K. Foreign Office said the video appears to be genuine.
US condemns 'barbaric murder'
U.S. President Barack Obama issued a statement late Saturday saying the U.S. "strongly condemns the barbaric murder" of Haines, vowing that Washington will work with Britain "and a broad coalition of nations ... to degrade and destroy this threat" to the world.
Cameron said Haines was "murdered in the most callous and brutal way imaginable by an organization which is the embodiment of evil."
The prime minister also gave his strongest signal yet that British military involvement could be stepped up beyond supplying ammunition and training to Kurdish forces.
"This organization poses a massive threat to the entire Middle East … so we will defeat ISIL through a comprehensive and sustained counter-terrorism strategy,” Cameron said.
Supporting Obama's promise of direct American military intervention in Iraq and Syria, Cameron said: "This is not about British combat troops on the ground, it is about working with others to extinguish this terrorist threat."
Haines worked for aid agencies in some of the world's harshest combat zones in recent years, including Libya during the 2011 civil war. He was taken hostage while working for the agency ACTED in March 2013, as he worked to bring aid to the Syrian people.
In a statement, his brother said he had a passion for humanitarian work.
"David was most alive and enthusiastic in his humanitarian roles," he said. "He was and is loved by all his family and will be missed terribly."
ACTED published a statement about the gruesome execution on its website shortly after the footage surfaced.
Islamic State militants have beheaded American journalists Jim Foley and Steven Sotloff in recent weeks.
The terrorist group said the Americans were killed in retaliation for U.S. strikes on their positions in Iraq.
A well-known Islamic cleric, Sheikh Awat Naqshbandi, tells VOA's Kurdish Service that beheadings are in no way Islamic.
He said militants have no place in Islam, saying the religion bans what the militants are doing.