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.
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.
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 fighter said.
“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 another Western captive.
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.” BBC
