David Cameron will gather the Government's Cobra emergency committee today to assess progress in the fight against Islamic State (IS).
The Prime Minister vowed that the terrorists would face "justice" yesterday after video footage surfaced showing the murder of US hostage Peter Kassig. He had been captured in Syria in October last year while providing medical training and humanitarian aid to victims of the country's conflict.
Meanwhile, the hunt is continuing for an extremist with a British accent, nicknamed "Jihadi John", who has been carrying out the on-screen beheadings. He is reported to have been injured in an allied air strike against IS commanders in neighbouring Iraq.
There is also speculation over whether a man shown standing next to the killer in one video is 20-year-old Cardiff student Nasser Muthana.
In a statement to MPs, Mr Cameron said: "We will not be cowed by these sick terrorists. They will be defeated and they must face the justice they deserve.
"The threat is faced by countries right across the world. We must face it together."
Senior intelligence and military officials are expected to attend the Cobra meeting later.
Meanwhile, a video analysis commissioned by the Times appeared to suggest that Mr Muthana was not pictured in the latest video.
An analyst, who reportedly compared the image to previous photos of the student, said: "Based on the initial analysis we have done, we are going to eliminate him as being one and the same person."
His father, Ahmed Muthana, told the paper: "I looked at the line-up and I didn't recognise my son. One of them looked like him but the eyes and nose are different."