US President Barack Obama’s counter-terrorism adviser, Lisa Monaco, said the country was evaluating the video released by the ISIL group showing the brutal beheading of British hostage, Alan Henning.

Also, in a statement, the British Foreign Office said it was also working to verify the video.

The video released on the internet Friday purports to show an ISIL fighter beheading Henning, the fourth such killing carried out by the armed group now being targeted by US-led air strikes.

It begins with a news clip announcing British strikes against ISIL. The authenticity of the video has not been verified, though it was released in the same manner as other ISIL videos and the masked man sounded similar to the one who carried out the other killings.

“This is again yet another just very clear example of the brutality of this group, and why the president has articulated and is moving out in a comprehensive way to degrade and destroy ISIL,” Monaco said.

“Our hearts go out to the British aid worker who we believe is in that video, and to the remaining hostages and to their families.”

Henning, 47, nicknamed “Gadget”, had joined an aid convoy and was taken captive on December 26, shortly after crossing the border between Turkey and Syria.

The statement released by the British Foreign Office said, “If true, this is a further disgusting murder. We are offering the family every support possible; they ask to be left alone at this time.”

Britain has been supporting US military efforts against ISIL by using British forces to help with logistics and intelligence gathering, as well as recently taking part in air strikes in Iraq.

Dozens of Muslim leaders in Britain had urged ISIL to release Henning. His wife had said she had been given hope by “the outcry across the world” over her husband’s imprisonment.

Mohammed Shafiq, chief executive of the Ramadhan Foundation, a Muslim charity, called Henning “a British hero”.

His “barbaric killing is an attack against all decent people around the world”, Shafiq said.

David Cameron, UK prime minister, said Henning’s apparent slaying showed “how barbaric and repulsive these terrorists are”.

“Alan had gone to Syria to help get aid to people of all faiths in their hour of need. The fact that he was taken hostage when trying to help others and now murdered demonstrates that there are no limits to the depravity of these terrorists. We will do all we can to hunt down these murderers and bring them to justice,” Cameron said in a statement.

The full beheadings are not shown in the ISIL videos, but the British-accented, English-speaking masked man holds a long knife and appears to begin cutting his victims, who include American reporter James Foley, American-Israeli journalist Steven Sotloff, British aid worker David Haines and now Henning.

The video ended with a fighter threatening a man identified as an American named Peter Kassig.

“Obama, you have started your aerial bombardment of Sham [Syria], which keeps on striking our people, so it is only right that we continue to strike the neck of your people,” the masked man in the video said.

Kassig, a 26 year-old American, enlisted in the army in 2004 and became a Ranger, ultimately serving in the 75th Ranger Regiment, an army special operations unit.

According to his military record, Kassig trained at Fort Benning, Georgia in 2006 and deployed to Iraq from April to July 2007.