A newly-released video appears to show British photographer John Cantlie, who was captured by so-called Islamic State in Syria in 2012, for the first time in more than a year.
The photojournalist, who has fronted a series of videos for IS while a hostage, appears gaunt as he ridicules US attempts to destroy the terrorist group.
His supporters swiftly dismissed the film, which is dated Saturday but has not been verified, saying: "His words are not those of a free man." They also questioned when it was made.
The Foreign Office said it was "looking at the contents of this latest propaganda video".
The clip, which first appeared on Twitter, shows the journalist clad in black as he walks around a bomb-damaged building he claims is in the northern Iraqi city of Mosul.
Mr Cantlie's skills as a journalist have been exploited by IS in an attempt to lend credibility to propaganda films, including a series titled Lend Me Your Ears.
Journalist Hala Jaber tweeted: "Worth remembering that whatever John Cantlie says, he is & remains a hostage and that his words are not those of a free man."
Mr Cantlie's father Paul, 80, died from complications following pneumonia in 2014.