New ISIS video claims beheading of American hostage Peter Kassig The Islamic State terror group has claimed to have beheaded American hostage Peter Kassig, an aid worker and former Army Ranger, in a graphic new video. In the nearly 16-minute video uploaded to social networks on Sunday, a black-clad militant with his face concealed stands before a severed head that he claims is that of the U.S. aid worker. The authenticity of the footage has not been verified. National Security Council spokesperson Bernadette Meehan said in a statement that intelligence officials were "working as quickly as possible to determine its authenticity. "If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," Meehan said. The video was posted shortly after President Obama departed for Washington from the G-20 summit in Australia. White House Press Secretary Josh Earnest said Obama was briefed by National Security Adviser Susan Rice while in flight. A government official confirmed to Fox News that the National Counterterrorism Center is aware of the video and is reviewing the tape. Ed and Paula Kassig, Peter's parents, released a statement early Sunday saying they were aware of the reports of their son's death and were awaiting confirmation of their authenticity. They also asked that media outlets not post any images or video distributed by Islamic State, better known as ISIS. "We prefer our son is written about and remembered for his important work and the love he shared with friends and family," the statement read, "not in the manner the hostage takers would use to manipulate Americans and further their cause." The video also showed what appeared to be the mass beheading of more than a dozen captured Syrian soldiers, but did not show the beheading of the person identified as Kassig, 26. Showing the execution of the soldiers is a departure from previous videos, which did not depict the act of beheading. The soldiers' executioners are not wearing masks in the video and warn they will carry out similar actions outside the region. "This is Peter Edward Kassig, a U.S. 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 video. He speaks in an audible British accent despite his voice being distorted to make it more difficult to identify him. "We say to you, Obama ...you claim to have withdrawn from Iraq four years ago," the militant said.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," he said, apparently 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. The new video is longer than its predecessors and shows multiple hostages being executed, as opposed to concentrating on a single hostage's death. It also attempts to tie ISIS leader, Abu Bakr al-Baghdadi, to Usama bin Laden and Abu Musab al-Zarqawi, the founder of Al Qaeda in Iraq, from which Islamic State claims descent. Sky News reported that the man featured in the video spoke in English with a British accent. The Associated Press reported that his voice had been distorted to make him harder to identify. It was not immediately clear whether he was the same militant who has appeared in other beheading videos and has been referred to as "Jihadi John" in accounts given by former hostages of their captivity. British Prime Minister David Cameron said he was "horrified by the cold-blooded murder," saying that the Islamic State group has "again shown their depravity." The video identifies the militant's location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border. The urban setting is another departure from previous beheading videos, which were filmed in the remote desert of northeastern Syria. Kassig would be the fifth Western hostage killed by ISIS in less than three months, and the third American. Previous Western beheading victims were American journalists James Foley and Steven Sotloff, as well as Britons David Haines, a former Royal Air Force engineer, and Alan Henning, a taxi driver from northwest England. The group is also holding British photojournalist John Cantlie, who has appeared in several other videos released by the group functioning as a de facto spokesman. It is not clear when the video was filmed. Last month, a Twitter account linked to ISIS posted a message warning that Kassig had only days to live. Sources in the intelligence community told Fox News that the message was being tracked. ISIS has beheaded and shot dead hundreds of captives -- mainly Syrian and Iraqi soldiers -- during its sweep across the two countries, and has celebrated mass killings in a series of slickly produced but extremely graphic videos. The group has declared an Islamic caliphate in the areas under its control in Syria and Iraq, which it governs according to a harsh version of Shariah law. The U.S. began launching airstrikes in Iraq and Syria earlier this year in a bid to halt the group's rapid advance and eventually degrade and destroy it. A video released last month appeared to show Kassig, of Indianapolis, kneeling as a masked militant says he will be killed next, after Henning's purported beheading. Kassig had been held in Syria since October 2013. Kassig formed the aid organization Special Emergency Response and Assistance, or SERA, in Turkey to provide aid and assistance to Syrian refugees. He began delivering food and medical supplies to Syrian refugee camps in 2012 and is also a trained medical assistant who provided trauma care to injured Syrian civilians and helped train 150 civilians in providing medical aid. After he appeared in the video, Kassig's parents released a public plea for their son's release, which included claims that Kassig had converted to Islam while in captivity and taken the name Abdul Rahman. The release of the video comes approximately a week after Syrian friends of Kassig called for his release, also saying that he had converted to Islam and was trying to help those afflicted by the country's three-year-old civil war. One of the friends, Amjad al-Moghrabi, told reporters in the northern Lebanon city of Tripoli: "We are demanding the Islamic State to release him, if they know Islam. He is a Muslim and has not participated in what his country is doing," a reference to the airstrikes. Dr. Ahmad Obeid, a friend of Kassig, said, "our demand is to release him and to return to his family because as a person he helped us and we should ask for mercy for him." "He is, unfortunately, detained so we are calling for his freedom because he supported our cause and we cannot leave him and let them hurt him," Obeid said. ISIS has declared an Islamic caliphate in the areas under its control in Syria and Iraq, which it governs according to an extremely violent interpretation of Shariah law. On Sunday, the Islamic State group claimed a bombing at the Baghdad International Airport that wounded five people, saying it was trying to strike Americans there. No one was wounded in a passing United Nations convoy, the organization said. The fight against the militant group adds another layer to Syria's complex civil war, now in its fourth year, which began as an uprising against President Bashar Assad. The Islamic State group emerged from the remains of Al Qaeda in Iraq and spread to Syria, where it battled both government forces and rebel groups as it carved out its self-styled Islamic state. In June the group swept into northern Iraq, capturing about a third of the country, including the second largest city Mosul, and eventually prompting the U.S. to resume military operations in the country less than three years after withdrawing. In September the U.S. expanded the air campaign to Syria. Fox News' Catherine Herridge and The Associated Press contributed to this report. Click for more from Sky News.