According to BBC, US President Barack Obama has said the beheading of US journalist James Foley is "an act of violence that shocks the conscience of the entire world".
Mr Obama said the ideology of Islamic State (IS), the group which made a video of Mr Foley's killing, was "bankrupt" and would ultimately fail.
The group said Mr Foley's killing was revenge for US air strikes on its fighters in Iraq.
The US, the UN and other nations have expressed abhorrence at the video.
Mr Foley's mother Diane said he "gave his life trying to expose the world to the suffering of the Syrian people".
Mr Foley, 40, had reported extensively across the Middle East, working for US publication GlobalPost and other media outlets including French news agency AFP.
In a statement, GlobalPost asked for "prayers for Jim and his family".
In the video, titled A Message to America, a man identified as James Foley is dressed in an orange jumpsuit, kneeling in desert-like terrain beside an armed man dressed in black.
He gives a message to his family and links his imminent death to the US government's bombing campaign of IS targets in Iraq.
Clearly under duress, he says: "I call on my friends, family and loved ones to rise up against my real killers, the US government, for what will happen to me is only a result of their complacency and criminality."
Then the masked militant, who speaks with a British accent, delivers a warning to the US government: "You are no longer fighting an insurgency. We are an Islamic army and a state that has been accepted by a large number of Muslims worldwide.
"So any attempt by you Obama to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people."
After he speaks, the militant appears to start cutting at his captive's neck before the video fades to black.
His body is then seen on the ground.
Another captive, identified as American journalist Steven Sotloff, is shown at the end, with the warning that his fate depends on President Obama's next move.
Mr Sotloff was abducted in northern Syria a year ago.