BEIRUT (AP) -- An Islamic State group video released Sunday purports to show extremists beheading a dozen Syrian soldiers and ends with a militant claiming to have killed U.S. aid worker Peter Kassig, the latest slaughter proudly broadcast by the group on the Internet.

The video ends with the militant standing over a severed head he says belongs to Kassig. U.S. officials said they were working to determine the video's authenticity. Kassig's family said it was awaiting the outcome of the investigation.

"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 family said in a statement.

The Associated Press could not independently verify the footage, though it appeared on websites used in the past by the Islamic State group, which now controls a third of Syria and Iraq.

The video identifies the militants' location as Dabiq, a town in northern Syria that the militant group uses as the title of its English-language propaganda magazine and where they believe an apocalyptic battle between Muslims and their enemies will occur.

The high-definition video shows the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits. The main militant in the video who speaks to the camera has a British accent and warns that U.S. soldiers will meet a similar fate.

"We say to you, 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 You claim to have withdrawn from Iraq four years ago," the militant said. "Here you are: You have not withdrawn.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." A U.S.-led coalition is targeting the Islamic State group in airstrikes, supporting Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.

The militant's voice is distorted in the video. Previous videos featured a militant with a British accent that the FBI says it has identified, though it hasn't named him publicly.