An Islamic State video has appeared which purports to show the beheading of Steven Sotloff, a US journalist being held hostage by the militants.
Mr Sotloff, 31, disappeared in Syria in 2013. He appeared at the end of a video last month which showed fellow US journalist James Foley being killed.
A militant in the video also threatens to kill a hostage said to be British.
After Mr Foley’s death, Mr 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.
‘Arrogant policy’
The video shows a masked figure together with Mr Sotloff, who is dressed in an orange jumpsuit.
The masked man 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.
Mr 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.
“If there is a video that’s been released, 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”.
Mr 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.
Credit: BBC