The Islamic State group has released a graphic video in which a black-clad militant claimed to have beheaded US aid worker Peter Kassig, who was captured last year.

The militant was standing over a body, but it was not immediately possible to confirm that it was Kassig, 26, who was pictured in the video. US officials said they are working to determine the video’s authenticity and the Kassig family said they are awaiting the outcome of the investigation.

Here we are, burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive

The video, which was posted on websites used by the group in the past, appeared to be the latest in a series of blood-soaked messages warning of further brutality if the US and its allies do not abandon an air campaign in Iraq and Syria.

“This is Peter Edward Kassig, a US citizen, of your country; Peter who fought against the Muslims in Iraq, while serving as a soldier,” the militant says near the end of the 16-minute video. He speaks in an audible British accent despite his voice being distorted to make it more difficult to identify him.

The video identifies the militant’s location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border.

The video also shows what appears to be the mass beheading of several Syrian soldiers captured by the group.

The militants warn that US soldiers will meet a similar fate.

“We say to you, [US President] Obama... you claim to have withdrawn from Iraq four years ago,” the militant said.

“Here you are: you have not withdrawn. Rather, you hid some of your forces behind your proxies,” he said, referring to Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.

“Here we are, burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive.”

Kassig, a former US Army Ranger, was providing medical aid to Syrians fleeing the civil war when he was captured inside Syria on October 1, 2013.

His friends say he converted to Islam in captivity and took the first name Abdul-Rahman.

Previous videos have shown the beheading of two American journalists and two British aid workers.

The latest video did not show the person identified as Kassig being beheaded. And unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else.

The group also holds British photojournalist John Cantlie, who has been shown in several videos delivering long statements in English on the group’s behalf, perhaps under duress.

Kassig’s family said in a statement they were aware of the reports of the video and were awaiting confirmation from the US government.

“The family respectfully asks that the news media avoid playing into the hostage takers’ hands and refrain from publishing or broadcasting photographs or video distributed by the hostage takers,” they said.

“We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause.”

The White House said the US intelligence community was working to determine the authenticity of the video. National Security Council spokeswoman Bernadette Meehan said that if the video is authentic, the White House would be “appalled by the brutal murder of an innocent American”.

The White House expressed its deepest condolences to Kassig’s family and friends, Meehan said.

Sign up to our free newsletters

Get the best updates straight to your inbox:
Please select at least one mailing list.

You can unsubscribe at any time by clicking the link in the footer of our emails. We use Mailchimp as our marketing platform. By subscribing, you acknowledge that your information will be transferred to Mailchimp for processing.