Second video of beheading is put online.
ISLAMIC militants have posted an online video which claims to show the beheading of US journalist Steven Sotloff, who went missing in Syria last year.
The Islamic State group said Mr Sotloff's killing was retribution for continued US air strikes targeting its fighters in Iraq.
It killed American journalist James Foley last month in the same manner and threatened to kill another hostage.
Mr Sotloff, 31, who freelanced for Time and Foreign Policy magazines, was last seen in Syria in August 2013 until he appeared in a video released online last month by the IS group that showed the beheading of Mr Foley.
Dressed in an orange jumpsuit against the backdrop of an arid Syrian landscape, Mr Sotloff was threatened in that video with death unless the US stopped air strikes on the group in Iraq.
In the video distributed last night and entitled "A Second Message to America," Mr Sotloff appears in a similar jumpsuit before he was purportedly beheaded by an IS fighter.
The Associated Press could not immediately verify the video's authenticity. Unlike Mr Foley's beheading, which was widely shared on Twitter accounts affiliated with the Islamic State group, the video purporting to show Mr Sotloff's killing was not immediately posted online.
In Washington, State Department spokeswoman Jen Psaki said US intelligence analysis will "work as quickly as possible" to determine if the video of the beheading is authentic.
"If the video is genuine, we are sickened by this brutal act, taking the life of another innocent American citizen," Ms Psaki said. "Our hearts go out to the Sotloff family and we will provide more information as it becomes available."
Ms Psaki said it's believed "a few" Americans are believed to still be held by the Islamic State but would not give any specifics.
The fighter who beheads Mr Sotloff in the video called it retribution for President Barack Obama's continued air strikes against the group in Iraq.
"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State ... despite our serious warnings," the fighter said. "So just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."