A video released onto the internet claims to show that Islamic State militants have killed the captured US aid worker Abdul-Rahman Kassig.
In the video, a masked militant stands over a severed head said to be that of Kassig.
It was not immediately possible to confirm the authenticity of the video, which also shows a group of Syrian troops being killed.
Kassig, also known as Peter, was captured last year.
In October, an IS video of the killing of British aid worker Alan Henning had ended with a threat to Kassig.
The US National Security Council said the intelligence community was working as quickly as possible to determine the latest video's authenticity.
"If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," NSC spokesperson Bernadette Meehan said.
Kassig's parents, who live in the US state of Indiana, last month released extracts of a letter written by their son, in which he told of the strains of captivity.
"This is the hardest thing a man can go through, the stress and fear are incredible," the aid worker wrote.
"They tell us you have abandoned us and/or don't care but of course we know you are doing everything you can and more.
"Don't worry Dad, if I do go down, I won't go thinking anything but what I know to be true. That you and mom love me more than the moon and the stars."
Kassig, 26, was a former US Army Ranger who served in the Iraq War.
He later trained as an emergency medical technician, travelling to Lebanon in May 2012 to work in border hospitals treating Palestinian refugees and later those fleeing the Syrian conflict.
Later that year he founded the Special Emergency Response and Assistance (Sera) organisation and subsequently moved its base of operations to southern Turkey, near the border with Syria.
Kassig sourced and delivered supplies to camps on both sides of the Syrian border, and helped to treat civilian casualties and train medics inside Syria.
He was undertaking a project for Sera when he was captured in October 2013 en route to eastern Syria.
