US President Barack Obama making a statement about the execution of American journalist James Foley by ISIS terrorist in Iraq denounced the act as one that "shocked the conscience of the world." Getty Images

US officials have revealed that they failed in a secret mission to free US reporter James Foley and other American hostages in Syria.

The Pentagon said the operation involved air and ground components and was focused on a particular captor network within the Islamic State outfit but did not succeed.

"Unfortunately, the mission was not successful because the hostages were not present at the targeted location," Pentagon spokesman Rear Adm John Kirby told CNN.

The admission follows the video footage by IS showing the beheading of Foley, claiming that it was retaliation for US air strikes against its fighters in Iraq.

Undeterred by the video and the threat issued by the IS, American warplanes have carried out at least 14 airstrikes against IS targets.

Foley's family had received an e-mail a week ago, saying he would be killed. "The message was vitriolic and filled with rage against the United States. It was deadly serious," said Philip Balboni, CEO of the online publication GlobalPost, which employed Foley.

Foley was kidnapped in Syria in November 2012.

US President Barack Obama has spared no words in condemning the act as one that shocked the "conscience of the world" and likened the militants to cancer.