James Foley

The Islamic State's (IS) beheading of a US journalist and its threat to "destroy the American cross" suggests it has gained enough confidence after seizing large areas of Iraq and Syria to take direct aim at US targets.

On Tuesday night, the IS released a video purporting to show one of its fighters, who spoke English with a British accent, beheading reporter James Foley, who was kidnapped in Syria nearly two years ago.

The video, titled "A Message To America," also showed images of another US journalist, Steven Sotloff, whose life the IS said depended on how the US acts in Iraq. It came following nearly two weeks of US air strikes that have pounded militant positions and halted the advance of Islamic State.

The gruesome video presented US President Barack Obama with bleak options that could define US involvement in Iraq and the public reaction to it, potentially dragging him further into a conflict he built much of his presidency on ending.

US National Security Council spokeswoman Caitlin Hayden said Wednesday that analysis of the video by the US intelligence community indicated that the video is authentic.

Obama was set to make a statement later Wednesday.

Other political leaders were swift to react.

British Prime Minister David Cameron interrupted his holiday to return to London to lead the hunt to identify the man shown killing Foley.

British Foreign Secretary Philip Hammond said he was not surprised to hear the British accent and that large numbers of British nationals are fighting in Iraq and Syria.

France said it wanted the permanent members of the UN Security Council and regional countries, including Arab states and Iran, to coordinate action against the IS.

Germany and Italy said they were ready to send arms to bolster the military capabilities of Iraqi Kurds fighting the IS in northern Iraq.

The beheading came as a surprise because the IS had seemed focused on proclaiming a caliphate in the parts of Iraq and Syria it controls, marching on Baghdad and redrawing the map of the Middle East.

Foley, 40, was kidnapped by armed men on November 22, 2012, in northern Syria while on his way to the Turkish border, according to GlobalPost, a Boston-based online publication where Foley had worked as a freelancer. He had reported in the Middle East for five years and had been kidnapped and released in Libya.

Sotloff, who appeared at the end of the video, went missing in northern Syria while reporting in July 2013.

On Facebook, Foley's mother Diane Foley said: "We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people.

"We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world."