The United States said Sunday it was working as quickly as possible to confirm the authenticity of a video showing the beheading of US aid worker Peter Kassig by Islamist militants.
"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," National Security Council spokeswoman Bernadette Mehan said.
The Islamic State group on Sunday released a video claiming the beheading Kassig in a warning to Washington as it prepares to send more troops to Iraq.
The same video showed the simultaneous beheadings of at least 18 men described as Syrian military personnel, the latest in a series of mass executions and other atrocities by IS militants.
In the undated video, a man who appears to be the same British-accented jihadist who beheaded previous Western hostages stands above a severed head.
"This is Peter Edward Kassig, a US citizen of your country," the black-clad masked executioner says, urging US President Barack Obama to send more troops back to the region to confront the IS group.