David Cameron has ramped up the case for extending RAF air strikes against Islamic State (IS) targets in Syria in the wake of the Paris terror attacks.
The Prime Minister said that the jihadists - who have claimed responsibility for the bloody wave of assaults in the French capital - represented a "direct and growing threat" to the UK.
In a Commons statement, he told MPs that IS needed to be dealt with in its Syrian stronghold of Raqqa - the "snake's head" - where its terror campaigns were planned and orchestrated.
"After the horror must come our resolve and determination to rid the world of this evil," he said.
The statement came as France issued an unprecedented demand for its European Union allies to support its military action against IS - also referred to as Isil, Isis or Daesh.
The French government invoked a never-before-used article of the EU's Lisbon Treaty which requires member states to provide "aid and assistance by all the means in their power" to a member that is "the victim of armed aggression on its territory".
French defence minister Jean-Yves Le Drian said EU partners could help "either by taking part in France's operations in Syria or Iraq, or by easing the load or providing support for France in other operations".
While Mr Cameron said that he would look very carefully at what Paris was saying, he made clear that he was determined to do all he could to support "our brothers and sisters" in France.
The Prime Minister said he would be setting out a "comprehensive strategy" for dealing with IS - including air strikes in Syria - in a response to a report by the cross-party Commons Affairs Committee which raised a series of concerns about the prospect of further UK military intervention.
"It is in Syria, in Raqqa, that Isil has its headquarters and it is from Raqqa that some of the main threats against this country are planned and orchestrated. Raqqa, if you like, is the head of the snake," Mr Cameron said.
"We face a direct and growing threat to our country and we need to deal with it not just in Iraq but in Syria too. Our allies are asking us to do this and the case for doing so has only grown stronger after the Paris attacks."
No 10 said there was no date or timetable for a Commons vote on military action in Syria, but the strategy would be set out by the end of the month.
The move comes amid signs that growing numbers of Labour MPs could defy Jeremy Corbyn and back military action - offsetting the Tory rebels who have made clear they would vote against.
The Labour leader has infuriated many of his own MPs with his response to the Paris attacks - including a suggestion he was "not happy" with the idea the security forces could follow a "shoot-to-kill" policy in the event of a similar attack in the UK.
A series of Labour backbenchers stood up in the House to voice their support for Mr Cameron's position and criticise their own leader.
Earlier the Ministry of Defence released details of the latest RAF air strikes in Iraq carried out on Monday by RAF Tornado GR4 fighter bombers in support of a Kurdish offensive in the north of the country.
"There was heavy cloud, which may have encouraged the terrorists to assume that they were safe from air attack but, working very closely with the Kurdish forces, the GR4s were able to guide a Paveway on to a large group of over 30 Daesh terrorists who were massing for a counter-attack; the Kurdish unit subsequently reported that the air strike had been highly effective," it said.
The previous day, an unmanned RAF Reaper drone provided "surveillance support" to a wave of French air strikes on Raqqa, carried out in retaliation for the Paris attacks.