LONDON - Prime Minister David Cameron said Sunday after an emergency meeting of his military and security chiefs that the Islamic State of Iraq and Syria terrorists pose a "massive" security threat that cannot be ignored.
"They are not Muslims, they are monsters," Cameron said after hailing executed British aid worker David Haines as a "British hero."
He did not announce new military measures but said British forces would continue offering logistical help to U.S. forces and that counterterrorism efforts will increase because the Islamic State of Iraq and Syria (ISIS, also known as ISIL) is planning attacks against Britain.
"We are a peaceful people. We do not seek out confrontation," Cameron said. "But we need to understand: we cannot ignore this threat to our security and that of our allies. There is no option of keeping our heads down that will make us safe."
The beheading of Haines - and ISIS' explicit threat against a fellow British captive - has prompted increased calls for Britain to take part in airstrikes against the militants, who have seized large swaths of territory in Syria and Iraq.
Cameron's statement did not appear to change much about London's policy towards ISIS, but the prime minister is under increasing pressure from within his own party to take more aggressive military action, The Guardian reports.
Many British politicians have been against using their military to involve itself in conflicts after the Iraq war, according to The Guardian.
Last year, Cameron lost a vote which called for the country to be prepared to join strikes against Syria.
Haines devoted his life to helping civilians in war zones and that is how he should be remembered, his family said Sunday as they grieved his death at the hands of ISIS.
Haines - the third Westerner beheaded in recent weeks by ISIS - had entered humanitarian work with enthusiasm, his brother Mike Haines said in a statement.
"He helped whoever needed help, regardless of race, creed or religion," he said of his brother.
British officials later released the name of a second U.K. hostage being threatened, identifying him as Alan Henning.
The 44-year-old Haines was kidnapped in Syria in March last year when he was working for the French aid group Agency for Technical Cooperation and Development, or ACTED, to help victims of the fighting there.
He had also worked for groups such as Handicap International, which helps the disabled during conflicts, and Nonviolent Peaceforce, which sends unarmed peacekeepers into conflict zones. He had previously been in Libya during its civil war and South Sudan. Mike Haines said he had also worked for the United Nations in the Balkans "helping people in real need."
The leader of Bosnia's Islamic Community, Husein Kavazovic, called on fellow Muslims Sunday to "show resolve to stop the murderers." He said Haines' family can be proud of his legacy.
Henning is also shown briefly on camera in the video and also threatened with death. British officials and Henning's family had asked the media not to publish his name out of concern for his safety, but that request was dropped Sunday afternoon. The family has also released a photo of Henning.
Britain's Foreign and Commonwealth Office said it saw no reason to doubt the authenticity of the video, which followed other videos that showed the beheadings of two U.S. journalists.
ISIS extremists had threatened Haines' life in an earlier video released nearly two weeks ago.
The masked man shown in the latest video, which ends with footage of Haines' body, seems similar to the man in the earlier beheading videos. The tall man speaks with a similar British accent and - as in the two earlier videos - threatens the captive with a large knife brandished in his left hand.
The video shows Cameron condemning ISIS. The man with the knife then condemns Britain's support of American action against the group and says the captive must pay with his life.
Cameron said Sunday the British public is sickened by the fact that the man on camera may be a British citizen.
Haines' family had issued a plea to his captors the day before the latest beheading video was released. They urged the hostage-takers to contact them. The family said ISIS had ignored earlier attempts to open communications.
British officials had said they were doing everything possible to protect Haines. An earlier rescue bid led by U.S. forces had failed, however, and it is not clear Western agencies know the precise location of the hostage-takers.
According to The New York Times, both Britain and the U.S. are among the only countries in the world which do not offer concessions when dealing with kidnappings by terrorist groups.
"We don't pay ransoms to terrorists when they kidnap our citizens," Cameron told Sky News earlier this month.
Mike Haines said his brother had joined the military as an aircraft engineer with the Royal Air Force after attending school and working for the Royal Mail.
He later got involved in humanitarian work and was "most alive and enthusiastic" when involved with such missions, Mike Haines said.
"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," Mike Haines said. "He was and is loved by all his family and will be missed terribly."
Haines had a teenage daughter in Scotland from a previous marriage and a four-year-old daughter in Croatia with his current wife, Dragana, who did not comment Sunday morning on the news of the killing.
British officials kept news of Haines' abduction out of the public eye for security reasons until ISIS showed him in a video nearly two weeks ago.
President Barack Obama said after the killing that the United States would stand with Britain in an expanded effort against the terror group.
"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," he said.
U.S. Secretary of State John Kerry has been pressuring allies ahead of a conference Monday to show a united front, especially from majority-Muslim nations, saying nearly 40 countries agreed to contribute to a worldwide fight to defeat the militants before they gain more territory in Iraq and Syria.
The White House said Sunday it would find allies willing to send combat forces - something the United States has ruled out - but that it was too early to identify them. The U.S. has so far been alone in carrying out airstrikes.
Several Arab countries offered to conduct airstrikes against ISIS, according to a State Department official traveling with Kerry who briefed reporters on condition of anonymity to discuss diplomat developments during his trip.
Germany and France, which is the international conference Monday to combat ISIS, also condemned Haines' killing.
"The odious assassination of David Haines shows once more the need for the international community to mobilize against the base and cowardly Daesh," French President Francois Hollande said, using the group's Arabic acronym.
German Foreign Minister Frank-Walter Steinmeier called the Haines killing "an abhorrent act of barbaric violence beyond all limits of human civilization" and said the Paris meeting comes at the right time.
There needs to be "internationally agreed action to effectively stop the flow of fighters and money," he said.