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, was abducted 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 latest video also threatens to kill a British hostage.
Mr Sotloff’s family said they were aware of the video and were “grieving privately”.
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.
President Barack Obama has ordered the deployment of another 350 troops to Baghdad to protect US diplomatic facilities, the White house has said.
This second execution video from IS is significant, even though it was largely expected and dreaded. It shows that the recent US air strikes which have halted IS’s lightning advance across northern Iraq are causing the organisation real damage, upsetting its plans to extend by force its rule into Kurdistan.
Unable to hit back militarily against America’s jets, Islamic State has responded with a form of information warfare that it knows will horrify most people in the West.
Secondly, by threatening to murder a British hostage, IS shows it makes little or no distinction between the US and Britain as its enemy. This is despite Britain so far restricting itself to dropping aid to refugees and flying in supplies to the Kurdish military, leaving air strikes to the Americans.
The video, entitled “A second message to America”, is about two-and-a-half minutes long and was apparently recorded in a desert.
It appears to have been filmed after Mr Foley’s, though it is impossible to determine the exact timing.
It shows a masked figure together with Mr Sotloff, who is dressed in an orange gown.
Mr Sotloff reads out a text addressed to Mr 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?