Conservative MPs have been questioned by Downing Street about their potential support for air strikes in Iraq and Syria.
A source told the Telegraph the mood amongst Tory MPs is "hardening" towards direct military action.
David Cameron is facing growing calls to join the United States in launching targeted action against Islamic State extremists.
Mr Cameron said he is "not ruling anything out" in regards to steps to combat IS, who are holding a British hostage.
I've been told at the Nato summit that the intelligence services know exactly who 'Jihadi John' is - or at least are 99.9% certain.
The British aid worker being held by Islamic State is David Haines, a 44-year-old father-of-two from Perth, Scotland.
In a video statement which claims to record his final words US hostage and journalist Steven Sotloff blames US foreign policy for his death.