The Islamic State militants who released a video Saturday purportedly showing the beheading of aid worker David Haines identified a second Briton named Alan Henning as their next victim, multiple reports said. In the footage, a masked man said Henning would be killed if United Kingdom Prime Minister David Cameron continued military action against the organization formerly known as ISIS.

“If you, Cameron, persist in fighting the Islamic State, then you, like your master Obama, will have the blood of your people on your hands,” the masked man said after Haines’ alleged execution. The militant then held a knife to another jumpsuit-clad individual, identified as Henning.

The British Foreign Office has yet to verify the authenticity of the purported Haines execution video or the identity of the second man. However, unconfirmed reports by Eliot Higgins -- a British "citizen journalist" who writes under the pseudonym “Brown Moses” -- and Dutch journalist Harald Doornbos identify Henning as another aid worker and British citizen. Moreover, Syrians who claim to recognize Henning wrote on Twitter he was kidnapped while providing aid to refugees from the country’s civil war.

If confirmed, Haines would be the third individual beheaded by ISIS since August, along with American journalists James Foley and Steven Sotloff. Each time, the militant group released a video that displayed the graphic execution of a man with ties to Western powers, followed by a threat against the next person to be killed if the West did not end hostilities against the Islamic State.  

Cameron condemned Haines’ apparent execution Saturday in a series of posts to his Twitter account. “The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude,” Cameron wrote. “We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes.”