American officials said the autheticity of the video is being checked by the US intelligence.
The video titled "A second message to America," shows the beheading of Sotloff, who is seen saying that he is "paying the price" for US intervention in Iraq.
It also threatens the life of British captive David Haines.
The news broke out during the daily White House news conference.
"I have not seen those reports today. I think that may have just happened in the last few minutes while I have been standing up here," White House Press Secretary Josh Earnest, told reporters at his daily news conference.
"This is something that the administration has obviously been watching very carefully, since this threat against Mr Sotloff's life was originally made a few weeks ago. Our thoughts and prayers, first and foremost, are with Mr Sotloff and Mr Sotloff's family and those who worked with him," he said in response to a question.
The US has dedicated significant time and resources to trying and rescue Sotloff, he said.
"I am not in a position to confirm the authenticity of that video or the reports at this point, obviously, since I just walked out here," the official said.
"But if there is a video that has been released, it is something that will be analysed very carefully by the US government and our intelligence officials to determine its authenticity," Earnest said.
Last month, IS have posted a video showing the beheading of journalist James Foley, prompting global outrage and strong condemnation.
The 40-year-old journalist had been reporting in Syria when he was abducted in November 2012.
National Security Council spokesperson Bernadette Meehan said: "We have seen a video that purports to be the murder of US citizen Steven Sotloff by the Islamic State. The intelligence community is working as quickly as possible to determine its authenticity."
"If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends," said Meehan.
"We will provide more information when it is available."