The Islamic State (IS) has released a video online purporting to show the beheading of US journalist James Foley, who went missing in Syria in 2012.

The jihadist militant group said the killing was revenge for US air strikes against its fighters in Iraq.

Foley's mother Diane said on Facebook she was proud of her son: "He gave his life trying to expose the world to the suffering of the Syrian people."

The White House said if the video was genuine, the US would be "appalled".

Foley has reported extensively across the Middle East, working for the US publication Global Post and other media outlets including the French news agency AFP.

'Second prisoner'

In the video, titled A Message to America, a man who appears to be James Foley is dressed in an orange jumpsuit, kneeling in desert-like terrain beside an armed man dressed in black.

The man identified as James Foley speaks, giving a message to his family and linking his imminent death to the US government's bombing campaign of IS targets in Iraq.

Then the masked militant, who speaks with a British accent and identifies himself as an IS member, describes Mr Foley as an American citizen and says: "As a government you have been at the forefront of aggression towards the Islamic State."

After he speaks, the militant appears to start cutting at his captive's neck before the video fades to black.

Another captive, identified as American journalist Steven Sotloff, is shown at the end, with the warning that his fate depends on President Barack Obama's next move.

Mr Sotloff was abducted a year ago in northern Syria, close to the border with Turkey.

In a statement, the Global Post asked for "prayers for Jim and his family", adding that it was waiting for the video to be verified.

UK Foreign Secretary Philip Hammond said the apparent beheading was an "appalling example of the brutality of this organisation".

Acknowledging that the militant appeared to be British, he said the involvement of a significant number of UK nationals in Syria in Iraq was "one of the reasons why this organisation represents such a direct threat to the UK's national security".

In a 2012 interview with the BBC, Foley described his motivation for covering conflicts.

"I'm drawn to the drama of the conflict and trying to expose untold stories," he said.

"There's extreme violence, but there's a will to find who these people really are. And I think that's what's really inspiring about it."

Foley was also briefly detained in Libya in 2011.

US officials confirmed that they had seen the video and were trying to establish its authenticity.

President Barack Obama had also been informed.

He has refrained from speaking publicly until the video can be formally authenticated, but the threat of retaliation against Americans is likely to complicate US involvement in Iraq, reports the BBC's Barbara Plett Usher in Washington.

White House National Security Council spokeswoman Caitlin Hayden said: "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."

The footage claimed to have been released by IS also shows another prisoner identified by the militants as an American reporter.

Mrs Foley, in her Facebook post, urged the militants to free any other hostages.

"Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world," she wrote.

The militant group has been accused of massacring hundreds of people in areas under their control in Iraq and also in eastern Syria.

The violence has displaced an estimated 1.2 million people in Iraq alone.

Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics.
(BBC)