American freelance journalist Steven Sotloff, 31, has been executed by the Islamic State, as shown in a new video released by the terrorist group that has established a caliphate inside Syria and Iraq.

The beheading of Sotloff was shown in a new video titled "A Second Message to America," which shows a British-sounding hooded man slicing a knife across the American journalist's throat until it is severed, and then putting the head atop the fallen body. Later in the video, the hooded terrorist threatened the life of British journalist David Haines.

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Before being beheaded, Sotloff calmly spoke to the camera saying he was paying the price for U.S. intervention in Iraq and Syria. It was believed the words were scripted for him.

The hooded executioner then addressed U.S. President Barack Obama: "I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State... despite our serious warnings," the terrorist said. "Just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."

Sotloff disappeared in August 2013 while reporting in Syria as a freelancer for Time and Foreign Policy magazines. Last month, a video showing the execution of American James Foley also showed the Islamic State militants threatening to execute him. 

A spokesman for Steven Sotloff's family in South Florida said they were awaiting authentication of the video.

"The family knows of the video and is grieving privately," Barak Barfi, the spokesman, said. 

Sotloff's mother, Shirley Sotloff, earlier released her own video pleading to the Islamic State to release her son, whom she described as an innocent journalist.

"Steven is a journalist who traveled to the Middle East to cover the suffering of Muslims at the hands of tyrants," Shirley Sotloff said. "Steven is a loyal and generous son, brother and grandson." Sotloff was an honorable man who always tried to help the weak, she said.

The National Security Council in Washington said U.S. intelligence agencies were trying to verify the authenticity of the video showing the execution. 

At the White House, spokesman John Earnest expressed the Obama administration's condolences to the Sotloff family. 

"This is something that the administration has obviously been watching very carefully since this threat against Mr. Sotloff's life was originally made a few weeks ago," Earnest said. "Our thoughts and prayers first and foremost are with Mr. Sotloff and Mr. Sotloff's family and those who worked with him."

British Prime Minister David Cameron described the new execution as "disgusting and despicable" and said he will be making an additional statement later.

TagsSteven Sotloff, ISIS, beheading, Video