A US secret military mission had "recently" tried but failed "to free a number of American hostages held in Syria", the Pentagon has revealed.

It did not say whether the US troops had also tried to rescue US journalist James Foley, a video of whose beheading by Islamic State appeared on Tuesday.

However, senior US officials - speaking on condition of anonymity - said this was the case.

IS said Foley's death was revenge for US air strikes on its fighters in Iraq.

US President Barack Obama condemned the killing as "an act of violence that shocks the conscience of the entire world".

He compared IS militants, who control large parts of Syria and Iraq to a "cancer" and said the group's ideology was "bankrupt".

The UN, UK and others have also 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".

The Committee to Protect Journalists said it was "extremely concerned for all journalists" still held by IS, describing Syria as "the world's most dangerous place to be a reporter".

'Firefight'

In a statement, the Pentagon said its operation "involved air and ground components and was focused on a particular captor network within ISIL" (the former name of IS).

"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."

Meanwhile, the senior Obama administration officials said that several dozen special troops had been dropped by aircraft into Syria earlier this summer to try to rescue US hostages, including Foley.

They added that the troops had been engaged in a firefight with IS militants, killing a number of them. No Americans were killed.

James Foley, 40, was seized in Syria in 2012.

He had reported extensively across the Middle East, working for US publication GlobalPost and other media outlets including French news agency AFP.

Source: BBC