A video released on Friday shows an Islamic State group fighter beheading a man purported to be a British hostage called Alan Henning.
This is the fourth killing carried out by the extremist group now targeted by US-led airstrikes.
The video also showed other beheading videos previously shot by the Islamic State group.
This new video ended with a militant threatening a man they identified as an American named Peter Kassig.
The militant group now holds the territory along the border of Syria and Iraq.
“Obama, you have started your aerial bombardment of Sham, which keeps on striking our people, so it is only right that we continue to strike the necks of your people,” the masked militant in the video said.
Caitlin Hayden, the National Security Council Spokesperson confirmed that Peter Kassig was actually held by Islamic State militants.
“At this point we have no reason to doubt the authenticity of the video released earlier today. We will continue to use every tool at our disposal – military, diplomatic, law enforcement and intelligence – to try to bring Peter home to his family,” Hayden said in a statement Friday night.
The AP could not verify the authenticity of the video, though it was released in the same manner as the previous videos, and the masked militant sound similar to the one who carried out other killings.
The British Foreign Office said it was working to verify the video.
“If true, this is a further disgusting murder,” the statement read. “We are offering the family every support possible; they ask to be left alone at this time.”
Britain has been working together with the U.S. against the Islamic State group with the British forces helping with logistics, intelligence gathering, as well as airstrikes.
“Alan had gone to Syria to help get aid to people of all faiths in their hour of need,” British Prime Minister David Cameron said in a statement. “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.”