Jihadist militant group Islamic State (IS) has posted a video online Tuesday purporting to show the beheading of US journalist James Foley.

IS said the execution was carried out in retaliation for US air strikes against its fighters in Iraq.

Foley, who had worked for the US publication Global Post and other media outlets including the French news agency AFP, went missing in Syria in 2012 after armed men kidnapped him.

"If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends," White House National Security Council spokeswoman Caitlin Hayden said in a statement.

Two weeks of US air strikes against IS positions have halted the advance of the militants which had captured a third of Iraq with little resistance until August.

According to Reuters coverage of the news, the video “injected an unpredictable element into US President Barack Obama’s deliberations on how far to proceed with US air strikes against IS targets in Iraq.” Presidential aides, however, said his vow not to put US combat forces on the ground in Iraq still held.

Another captive, identified as American journalist Steven Sotloff, was shown at the end of the video titled A Message to America, with the warning that his fate depends on Obama's next move. Sotloff was abducted a year ago in northern Syria.

In the video, a man identified as James Foley is standing on his knees beside a knife-toting man dressed in black.

Speaking with a British accent, the masked man issues a warning to the US government that "any attempt [...] to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people."

The BBC quoted British PM David Cameron as saying:"If true, the murder of James Foley is shocking and depraved."

British Foreign Secretary Philip Hammond acknowledged the masked man appeared to be British. He told the BBC that the involvement of a significant number of UK nationals in Syria and Iraq was one of the reasons why the IS "represents such a direct threat to the UK's national security",