A video has been circulated online purporting to show members of the Islamic State of Iraq and the Levant group beheading several Syrian soldiers and a US aid worker.

The footage, released on Sunday, showed the beheading of at least 12 people whom ISIL said were pilots and officers in Syrian President Bashar al-Assad's military.

The clip was circulated through credible social media accounts that have previously shared similar videos that were later verfied, Al Jazeera's Imran Khan, reporting from Baghdad, said.

But it is not yet possible to verify the authenticity of the footage, Khan 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, whom ISIL said was that of US aid worker Peter Kassing.

"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."

'Brutal muder'

The White House said the US intelligence community was working to determine the authenticity of the video.

If the video is authentic, the White House would be "appalled by the brutal murder of an innocent American," National Security Council spokeswoman, Bernadette Meehan, said.

In a highly choreographed sequence in the video, the self-declared jihadists marched at least a dozen prisoners said to be Syrian officers and pilots past a wooden box containing long military knives, each ISIL member taking one as they passed.

They then forced the men to kneel in a line and decapitated them.

Two American journalists and two British aid workers had already been beheaded by ISIL in what the group says are retaliatory killings for the US-led air campaign against its fighters in Iraq and Syria.

ISIL has also executed numerous Iraqis and Syrians. In October, the group  publicly executed Iraqi news cameraman Raad al-Azzawi and three civilians in northern Iraq, according to the journalist's relatives.

Kassig, a 26-year-old from the US state of Indiana, converted to Islam and changed his name to Abdul-Rahman Kassig.
(Al Jazeera)