Charities, Muslim groups and politicians are expressing outrage over the beheading of another British aid worker by Islamic State (IS) extremists.
A video purporting to show the killing of British aid worker Alan Henning has been released on YouTube, and highlighted on pro-IS Twitter feeds.
Mr Henning - a 47-year-old taxi driver from northern England - was taken hostage last December while taking part in an aid convoy to Syria.
David Cameron said Britain would do all it could "to hunt down these murderers and bring them to justice", the BBC reported.
The British prime minister said the killing showed "how barbaric and repulsive" IS was.
"My thoughts and prayers tonight are with Alan's wife Barbara, their children and all those who loved him," he said.
"Alan had gone to Syria to help get aid to people of all faiths in their hour of need."
President Barack Obama said the United States strongly condemned the brutal murder.
Mr Henning's wife Barbara had this week appealed for her husband's release, saying: "He is innocent."
An activist battling extremism, Hanif Qadir, from the Active Change Foundation, said IS went against everything in which Muslims believe.
"These people, instead of bowing down and begging for forgiveness from their creator, decide to kill somebody. On the contrary to what - everything Islam teaches. It's totally un-Islamic, these people do not represent Muslims and we should not call them the Islamic State, we should call it inhuman evil, evil individuals."
IS has previously released videos showing the apparent beheadings of two US journalists James Foley and Steven Sotloff, and British aid worker David Haines. It threatened to kill Mr Henning in footage showing the death of Mr Haines last month.
The video released today is yet to be verified, but it appears to show Mr Henning kneeling beside a militant, dressed in black, in a desert setting.
The footage ends with an IS fighter threatening a man they identify as an American aid worker, Peter Kassig.
BBC security correspondent Gordon Corera said the footage was similar to previous videos released by IS, though slightly shorter.
He said it included a reference to last week's vote by UK Parliament to authorise air strikes against IS in Iraq.
Like previous videos it features a militant with an apparently British accent, he added.
The UK Foreign Office said in a statement: "We are aware of the video and are working urgently to verify the contents.
"If true, this is a further disgusting murder.
"We are offering the family every support possible; they ask to be left alone at this time."