An Islamic State video has appeared, purportedly showing 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 latest video also threatens to kill a British hostage.
Mr. Sotloff's family said they were aware of the video and were "grieving privately".
Afteri Mr. Foley's death, Sotloff's mother appealed to ISIS leader, Abu Bakr al-Baghdadi, to save her son's life.
Josh Earnest, White House spokesman, said US officials were checking the reports.
The US has recently carried out dozens of air strikes against ISIS 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 was apparently recorded 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 gown. Sotloff reads out a text addressed to President Obama saying: "You've spent billions of US taxpayers' dollars and we have lost thousands of our troops in our previous fights 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 man who appeared to carry out the beheading of James Foley, then describes the act to be committed 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 with 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."
White House spokesman Josh Earnest: "Our thoughts and prayers are with Mr Sotloff and his family"
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, adding that he was 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".
Mr. Cameron's office said he has known for months that a Briton was among the hostages taken by Islamic State and has chaired meetings with high-level officials to discuss the situation.
But British officials said they have deliberately not commented on this while there has been what they call "strong family liaison" with relatives of the hostage.