David Cameron has described the latest Islamic State killings video, which features a British-sounding man and young boy, as "desperate stuff" from a group that is "losing territory" and "increasingly losing anybody's sympathy".
A spokesperson for the Prime Minister earlier said the released footage should be dismissed as propaganda.
The Foreign Office is examining the video which shows the apparently British militants along with the killing of five men accused of spying for the UK.
Mr Cameron was individually cited as an "imbecile" during the video by the gun-wielding jihadi with an English-sounding accent who condemned the RAF bombing campaign against IS in Iraq and Syria.
"We are examining the content of the video and the Prime Minister is being kept updated on that," his spokesperson said.
"It serves as a reminder of the barbarity of (Islamic State) and what the world faces with these terrorists. It is also clearly a propaganda tool and should be treated as such."
Despite multiple claims to the contrary, the man behind the mask in the latest IS propaganda video has not yet been identified.
A leading candidate emerges in the push to name the 'Brit' from the latest IS killings video - but his sister and mentor say it isn't him.
Henry Dare told ITV News he had not seen his grandson for three years since his daughter, a known jihadi, took him with her to Syria.