An Internet video uploaded Tuesday by Islamic State purports to show the beheading of U.S. journalist Steven Sotloff.
A masked figure in the video also issue a threat against a British hostage, a man the group named as David Haines, and warned governments to back off "this evil alliance of America against the Islamic State," the SITE monitoring service reported.
The purported killer addresses President Barack Obama, saying Sotloff's death was in retaliation for U.S. airstrikes against the militants.
The militant says, "Just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."
U.S. State Department spokeswoman Jen Psaki said officials are checking into the video's authenticity. If it's genuine, Psaki said, "we are sickened."
The images follow last month's video release of the gruesome beheading of U.S. freelance journalist Jim Foley.
Video appears authentic
A source familiar with the matter said that while U.S. officials have yet to formally confirm the validity of the video, it appeared to be authentic.
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