Jihadist group the Islamic State has claimed to have executed American journalist James Foley in revenge for US air strikes against its fighters and has threatened to kill a second reporter.

The Sunni Islamist group released a video on Tuesday showing a masked militant purportedly beheading Foley, who has been missing since he was seized by armed men in Syria in November 2012.

A second captive, introduced as US reporter Steven Sotloff, is shown alive, along with a warning that his fate rests on US President Barack Obama ordering a halt to military action against the jihadist group.

James Foley

The White House said US intelligence was studying the video, which is reportedly titled “A Message to America”.

“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,” National Security Council spokeswoman Caitlin Hayden said.

Labor senator David Feeney has described the news as “the latest in a ghastly and grotesque chain of atrocities”.

Unconfirmed photos and footage from the video have also been posted to Twitter.

Fellow journalists and Foley supporters are calling for a media blackout in order to curb the militants’ influence.

The only two weapons ISIS has is the propensity for violent, gruesome murder, and an internet connection. Take away one. #ISISmediaBlackout

— Hend (@LibyaLiberty) August 19, 2014

— with AAP