President Barack Obama condemned the "brutal" murder of British captive Alan Henning on Friday, saying the United States would bring those responsible to justice.
"Standing together with a broad coalition of allies and partners, we will continue taking decisive action to degrade and ultimately destroy ISIL," he said in a statement, referring to the Islamic State jihadist group that killed the aid worker.
In a video released by the group in response to US-led air strikes against it, a masked IS fighter beheads Henning and threatens a US captive, aid worker Peter Kassig.
The footage, almost identical to three previous execution films released by the group, inspired revulsion around the world and drew a stark warning from Obama and Britain's Prime Minister David Cameron.
"Mr Henning worked to help improve the lives of the Syrian people and his death is a great loss for them, for his family and the people of the United Kingdom," Obama said in the statement.
"Standing together with our UK friends and allies, we will work to bring the perpetrators of Alan's murder - as well as the murders of Jim Foley, Steven Sotloff and David Haines - to justice."
American reporters Foley and Sotloff, and Haines, a British aid worker, were all previously executed on camera.
National Security Council spokesperson Caitlin Hayden said the latest video appeared to be authentic.
"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," she said.