British Prime Minister David Cameron on Sunday praised slain British aid worker David Haines as a hero and pledged to continue working as part of an international coalition to bring to justice the perpetrators, who he said "are not Muslims, they are monsters."
The militant group Islamic State, also known as ISIS or ISIL, released a video Saturday showing his beheading. Britain's Foreign Office said the video appeared to be authentic.
"Today the whole country will want to express its deep sympathy for David Haines' family," Cameron said after chairing an emergency Cabinet meeting Sunday morning at the prime minister's official residence.
Cameron added that Britain and the rest of the world cannot ignore the threat Islamic State poses. "Step by step we must drive back, dismantle and ultimately destroy ISIL and what it stands for," he said.
Britain has partnered with the United States in its push to thwart the militant group by providing assistance to the Iraqi military and to Kurdish fighting forces, known as the peshmerga, in northern Iraq.
However, Cameron is under growing pressure over whether Britain will join in U.S. airstrikes against the Islamic State. The prime minister did not address the question of airstrikes in his statement Sunday.
In the execution video, the Islamic State militant, garbed in black with his face obscured, threatened Cameron over Britain's alliance with the U.S.
"Your evil alliance with America, which continues to strike the Muslims of Iraq … will only accelerate your destruction," he says. "And playing the role of the obedient lap dog, Cameron, will only drag you and your people into another bloody and un-winnable war."
The video followed the same format as previous ones that showed the slayings of U.S. journalists James Foley and Steven Sotloff. The militant speaks with a British accent and appears to be the same person as in the other video recordings.
At the end, the militant presents a second British hostage, Alan Henning, whom he also threatens to kill in response to the policies of Britain and its allies. Henning is a 47-year-old taxi driver from Salford near Manchester who traveled to Syria as a volunteer in a convoy delivering humanitarian aid.
At a news conference in Paris on Sunday, Frederic Roussel, director of development at the Agency for Technical Cooperation and Development, or ACTED, said the organization was stunned at Haines' beheading and would initiate an investigation Monday.
He said Haines, 44, was hired by the Paris-based aid organization to provide logistics for its operation to deliver humanitarian relief to civilians displaced by Syria's civil war. He was kidnapped 10 days after his arrival in Syria less than a mile from the Turkish border.
In response, ACTED evacuated its staff in Syria and in other conflict zones, the first time it had done so since it was founded two decades ago. Roussel said the targeting of humanitarian workers raises questions about how aid organizations can continue to offer assistance in dangerous regions.
He acknowledged an investigation will be a largely symbolic act.
Haines' brother Mike released a statement Sunday via Britain's Foreign Office saying, "David was most alive and enthusiastic in his humanitarian roles. 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."
Haines had also worked with aid groups in the Balkans, South Sudan and Libya, after a career as an aircraft engineer with Britain's Royal Air Force.
Surviving him are his wife and 4-year-old daughter in Croatia, where Haines lived before his kidnapping, and a 17-year-old daughter in Scotland from a previous marriage.
His killing was condemned by many in Britain's sizable Muslim community.
Qari Asim, imam of the Makkah Mosque in Leeds, said in a statement: "David Haines' burning passion and courage to help people in need in conflict zones will continue to inspire generations. His savage murder is not only an attack on an individual, but an attack on the whole humanitarian cause.
"It is a moral obligation on British Muslims to actively oppose ISIS' toxic ideology, especially when this is being promoted within Britain."
The British government estimates that at least 500 Britons have traveled to fight with extremist rebel groups in Syria.