A video has been circulated online purporting to show members of the Islamic State of Iraq and the Levant (ISIL) 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.

The US government believes that the video is authentic and Kassig is dead, the New York Times said on Sunday, quoting a senior US official.

In the video, a masked man said before he was shown beheading one of the men: “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.”

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.”

Khan said: “Peter had converted to Islam. He is a Muslim who died in the hands of Muslims. So it is a pretty controversial statement coming from ISIL.”

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

‘Brutal murder’

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.

Kimberly Halkett, reporting from Washington, said: “Peter’s family has released a statement, saying that they are in contact with the US government to understand if the video is authentic or not. They asked the international news media not to broadcast or publish any material provided by the hostage takers.”

British Prime Minister David Cameron said he was “horrified” over the video.

“I’m horrified by the cold-blooded murder of Abdul-Rahman Kassig,” Cameron wrote on Twitter on Sunday.

“ISIL have again shown their depravity. My thoughts are with his family.”

Two US 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.

Later on Sunday, the Syrian Observatory for Human Rights, said, in Syria’s al-Raqqa city, ISIL executed al least 13 men, who were arrested earlier this month after they were charged of collaborating with the Syrian regime.

The London-based rights group said, according to local sources, the Syrian nationals were arrested about a month ago, adding that ISIL found individual weapons and wireless devices in their possession.

ISIL has 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. Al Jazeera