A video has been circulating online purporting to show members of the Islamic State (IS) group beheading several Syrian soldiers and a US aid worker.
The footage, released Sunday, showed the beheading of a captured American aid worker, Abdul-Rahman Kassig, and at least 12 people who IS said were Syrian pilots and officers in President Bashar al-Assad's forces, Al Jazeera reported.
The aid worker was earlier known as Peter Kassig before he converted to Islam. He was kidnapped in Syria last year.
The clip was circulated through credible social media accounts that have previously shared videos that were later verified, an Al Jazeera correspondent said.
But it remains hard to verify the authenticity of the footage, he said.
"To Obama the dog of rome, today we are slaughtering the soldiers of Bashar and tomorrow we are slaughtering your soldiers and with god's permission we will break this final and last crusade," a masked man said before he was shown beheading one of the men.
The video also showed a bloodied head on the ground, which was that of Kassig, IS said in the video.
"This is Peter Edward Kassig, a US citizen of your country. Peter, who fought against Muslims in Iraq, was serving as a soldier in the American army, does not have much to say," a masked person standing near the head said.
"His previous cellmates have already spoken on his behalf."
Between August and October this year, the Islamic State has beheaded American-Israeli journalist Steven Sotloff, British aid worker Alan Henning, American journalist James Foley and British aid worker David Haines.
New York Times quoted a senior US official saying that the government believed the footage was authentic but had not officially confirmed. He said it was significantly different from the execution videos of four other Western hostages, whose televised deaths were carefully choreographed.
A National Security Council spokesperson said the US intelligence community was aware of the video and was "working as quickly as possible to determine its authenticity."
