ISIS Threatens To Execute British Aid Worker David Haines

In a video that appears to show the beheading of an American journalist, militants of the Islamic State group in Syria have threatened to take the life of another hostage.

The video, released Tuesday, purports to show the killing of U.S. reporter Steven Sotloff at the hands of an IS member. The video also includes a threat to behead a man the militants identify as David Cawthorne Haines, a British aid worker, if the U.S. continues its strikes against the extremist group.

Britain's Foreign Office has not yet confirmed whether the hostage is Haines.

According to The Washington Post, Haines was kidnapped in March 2013 in northeastern Syria, near the Turkish border.

Haines appears to have had years of experience in nongovernmental organizations and military environments. An official with the civilian peacekeeping group Nonviolent Peaceforce confirmed to NBC that Haines worked for the group in South Sudan in 2012. The official also told the network that Haines was "very familiar with insecure locations."

According to SITE Intelligence, the group that first obtained Tuesday's video, Haines appears in the footage wearing an orange Guantanamo-style jumpsuit alongside a masked militant dressed in black. The jumpsuit is similar to those worn by Sotloff and reporter James Foley, who was killed by Islamic State fighters last week.

"We take this opportunity to warn those governments that enter this evil alliance of America against the Islamic State to back off and leave our people alone,” a fighter in the video said, in what appears to be a reference to the closely aligned geopolitical priorities of the U.S. and the U.K.

British Prime Minister David Cameron described Sotloff's alleged murder as "an absolutely disgusting and despicable act [by] barbaric terrorists." Reuters reports that Cameron will meet with his security crisis team on Wednesday to discuss the situation.

According to The Daily Telegraph, Cameron had been briefed regularly on attempts to free the British hostage.

The Telegraph reports that the family of the hostage has been notified of the threat.