President Barack Obama's top counter-terrorism advisor Lisa Monaco said Washington was taking steps to confirm the authenticity of the tape.
"If it in fact proves to be authentic, it is yet another demonstration of the brutality of ISIL and our hearts go out to the British aid worker who we believe is in that video," she said, using an alternative acronym for the group.
"This is yet again another clear example of the brutality of this group and why the president has articulated and (is) moving out in a comprehensive way to degrade and destroy
The jihadist group earlier claimed responsibility for the murder of Henning, an aid worker, in a video showing his apparent execution.
In the video, almost identical to those released after three previous murders, a masked IS militant also displays a hostage he identifies as an American, Peter Kassig.
IS has previously released videos showing the beheadings of American journalists James Foley and Steven Sotloff, and another British aid worker David Haines.