An Islamic State video has appeared which purports to show the beheading of Steven Sotloff, a US journalist being held hostage by the militants.
Sotloff, 31, disappeared in Syria in 2013 and appeared at the end of a video last month which showed fellow US journalist James Foley being killed.
A militant in the latest video also threatens to kill a British hostage.
Sotloff’s family said they were aware of the video and were “grieving privately”.
After Foley’s death, Sotloff’s mother appealed to IS leader Abu Bakr al-Baghdadi to save her son’s life.
White House spokesman Josh Earnest said US officials were checking the reports.
The US has recently carried out dozens of air strikes against IS targets in Iraq.
BBC security correspondent Frank Gardner says the apparent beheading is a way for IS to get back at the US for its actions.
The video, entitled “A second message to America”, is about two-and-a-half minutes long and is apparently set in a desert.
It appears to have been filmed after Foley’s, though it is impossible to determine the exact timing.
It shows a masked figure together with Sotloff, who is dressed in an orange jumpsuit.
Sotloff reads out a text addressed to Obama saying: “You’ve spent billions of US taxpayers’ dollars and we have lost thousands of our troops in our previous fighting against the Islamic State, so where is the people’s interest in reigniting this war?”
The masked man, whose voice is similar to that of the executioner in the Foley video, then describes the act he is about to commit as retribution for the US air strikes.
“I’m back, Obama, and I’m back because of your arrogant foreign policy towards the Islamic State… despite our serious warnings,” the man says.
“We take this opportunity to warn those governments that enter this evil alliance of America against the Islamic State to back off and leave our people alone.”
The video ends with the militant threatening to kill a captive who is claimed to be British.
US state department spokeswoman Jen Psaki said: “If the video is genuine, we are sickened by this brutal act.”
Earnest urged caution about the veracity of the video. “Our thoughts and prayers, first and foremost, are with Mr Sotloff and Mr Sotloff’s family and those who worked with him,” he said.
“I’m not in a position to confirm the authenticity of that video or the reports. It’s something that will be analysed very carefully by the US government and our intelligence officials to establish its authenticity.”
UK Prime Minister David Cameron described the apparent beheading as an “absolutely disgusting, despicable act”.
‘Unbiased observer’
A friend of Sotloff, US film maker Matthew Van Dyke, told the BBC: “He was a complete professional and there was no reason for this to happen to him.
“He was an unbiased, impartial observer to the events, to tell the world about it and now he’s paid for it with his life.”
Sotloff was abducted near Aleppo in northern Syria in August 2013.
He had worked for Time magazine, Foreign Policy and the Christian Science Monitor, and reported from Egypt, Libya and Syria.
Friends said he had lived in Yemen for many years and spoke good Arabic.
At the time of his capture, his family chose not to go public with details, on the advice of officials.
Last month a video was released showing the beheading of Mr Foley.
Sotloff was shown at the end, as a militant gave a warning that his fate depended on President Obama’s next move.
The US launched has launched more than 120 air strikes in Iraq in the last month, in an attempt to help Kurdish forces curb the advance of Islamic State militants and protect minorities threatened by them.
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