Prime Minister David Cameron is summoning military and security chiefs for an emergency meeting Sunday in response to the beheading of a British hostage and a threat against another.
The meeting comes after Islamic extremists released a video showing the beheading of British aid worker David Haines and threatening another with death.
Mike Haines, the victim's brother, said David Haines had been murdered "in cold blood." Britain's Foreign and Commonwealth Office said it saw no reason to doubt the authenticity of the video.
Haines is the third Westerner beheaded in recent weeks by the Islamic State group, which has seized vast swaths of territory in Syria and Iraq. The first two were U.S. journalists.
Islamic State extremists had threatened Haines' life in an earlier video released nearly two weeks ago.
The 44-year-old aid worker's 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 IS 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.
Haines was kidnapped in Syria in March last year when he was working for the charity ACTED to help victims of the fighting there.
After his execution, Haines' family released a statement praising his passion for charitable work.
Mike Haines said his brother was "most alive and enthusiastic" when involved with humanitarian missions.
"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 of his late brother. "He was and is loved by all his family and will be missed terribly."
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.
France, which is holding an international conference Monday to combat IS, 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.
Some British lawmakers called for Britain to launch air strikes against Islamic State forces after the killing.