David Cameron has pledged to make the case for 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.
Mr Cameron told MPs Raqqa in Syria was the "snake's head" for IS and Britain should be attacking there to "rid the world of this evil".
The statement - which sets up a Commons showdown with Labour leader Jeremy Corbyn, who has made clear his opposition to military action - came as France issued an unprecedented demand for its European Union allies to support its military action against IS.
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".
Other developments included:
:: Police have been searching a flat and two hotel rooms they believe were used by the attackers, and inspecting a car rented by Salah Abdeslam, the suspected eighth surviving gunman who is now the subject of an international manhunt.
French authorities think the black Renault Clio was used by Salah and brother Brahim to ferry the terrorists to and from Belgium, where the attacks were planned.
:: Mohamed Abdeslam, another brother of Salah, has made a TV appeal urging him to turn himself in.
:: A French jihadist, Fabien Clain, has reportedly been identified as the voice in a recording issued by IS claiming responsibility for the atrocities.
:: Officials said 117 of the 129 victims have now been identified.
:: French warplanes have carried out fresh strikes against the IS stronghold of Raqqa in northern Syria, destroying a command centre and training centre.
:: Russia has also ramped up its assault on the extremist group, dropping bombs and firing cruise missiles from a submarine.
:: Belgium's government has raised its terror threat level and cancelled a football match between the national team and Spain.
:: US secretary of state John Kerry has held talks with French president Francois Hollande in Paris, arguing that Syria was on the verge of a "big transition" after international talks in Vienna at the weekend.
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 personally be setting out a "comprehensive strategy" for dealing with IS - also referred to as Isil or Daesh - 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 it.
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.
Julian Lewis, the Tory MP who chairs the foreign affairs committee, said: "What we need is a coherent campaign plan, endorsed by the chiefs of staff, involving the use of regional Muslim forces which are not Islamist, and which are ready to remain as an occupying power for years to come.
"Air strikes will not be decisive unless in support of credible non-Islamist ground forces of this type, and co-operation with the Russians will also be required."