David Cameron has warned that Islamic State (IS) must be destroyed in its Syrian heartland as Labour signalled it was ready to back further military action in the wake of Tunisia beach attack.
In the Commons, Defence Secretary Michael Fallon began laying out the case for extending RAF air strikes into Syria, telling MPs that was where IS - or Isil as it is also referred to - organised and directed its operations.
He made clear however that the Government would not stage a new vote of MPs unless it was clear there was "some consensus" across the House for widening the existing RAF operations against IS in Iraq.
With senior Conservative MPs continuing to their voice their opposition, the support of Labour is crucial if ministers are to be sure of a Commons majority.
In a reversal of their previous position last September - when Ed Miliband made clear they would block strikes on Syria - shadow defence secretary Vernon Coaker said Labour would "carefully consider" any new proposals from the Government.
Nevertheless, with a leadership contest ongoing, the signs are that Mr Cameron will wait until September when a successor to Mr Miliband is in place before going back to the Commons with a new motion.
The Prime Minister is determined to avoid a repeat of the 2013 vote on military action against Syrian president Bashar al-Assad when he suffered a damaging Commons defeat at the hands of Labour and rebel Conservative MPs.
Mr Cameron's official spokeswoman said there was a need for "more thought, more deliberation, more time" before any decision was taken, although she stressed that he still believed that ultimately IS had to be dealt with in Syria.
"The PM has long thought that Isil poses a threat to Britain and Isil needs to be destroyed in Syria as well as in Iraq," she said.
"The PM's views haven't changed. What has changed is the growing evidence that Isil represents a threat to Britain and our national security."
With some Tories questioning the legal basis of strikes on Syrian territory, she said the Government was satisfied a case for action had been established last year when the US-led coalition commenced air strikes.
She pointed out that the UK was already engaged, providing reconnaissance and air-to-air refuelling capacity for the Syrian operation.
In the commons Mr Fallon stressed the Government was determined to ensure its actions did not offer any "succour" to Assad whose forces are pitted against IS in the brutal Syrian civil war.
However he said the United States and other allies were already carrying out air strikes in Syria and that it was right for Britain to consider whether it could do more in the light of the casualties suffered by British tourists in Tunisia.
"This is of course though a new Parliament and it is for all members to consider carefully how best to tackle Isil - an evil caliphate that does not respect state boundaries," he said.
"We want Assad to go but we are equally clear that Isil operations in Iraq and elsewhere - and probably in Libya - are being directed from northern Syria.
"We already have American air strikes being carried out in northern Syria, we have air strikes being carried out by other Gulf countries. We have air strikes being carried out by Canadian aircraft that are, of course, helping to keep our streets safe as well."
Setting out Labour's terms for support, Mr Coaker said ministers would need to demonstrate the purpose and legal basis of any new action as well as the backing of other states in the region.
"We stand ready to work with the Government to defeat Isil and will carefully consider any proposals that the Government decide to bring forward," he said.
Mr Fallon was however challenged by senior Conservatives, underlining the Government's need for cross-party support.
Julian Lewis, the newly-elected chairman of the Commons Defence Committee, said that having wanted to remove Assad without helping the extremists in 2013, ministers now wanted to remove IS without helping Assad.
"These two things are incompatible. It is a choice of evils. Which does he think is the lesser of those two evils?" he demanded.
Former cabinet minister John Redwood said a terrorist who plotted murders in the UK was arrested, prosecuted and imprisoned.
"If that same terrorist goes to Iraq we then try to hunt them down and kill them and blow up the building they are living in," he said. "How does that help create a rule of law or democratic pressures in Iraq?"
Mr Cameron discussed the IS threat with the Crown Prince of Abu Dhabi Mohammed bin Zayed Al Nahyan, at Number 10.
A Downing Street spokesman said: " Both leaders agreed that Isil, and the threat it poses, must be defeated. The Prime Minister reiterated the support the UK was providing to the Iraqi government including carrying out air strikes as part of the coalition and providing training to the Iraqi security forces where the UK most recently sent an additional 125 trainers."
The two leaders said Iraqi prime minister Haider al-Abadi "must ensure he was a leader for all of Iraq's people".
On Syria, they agreed that Assad " was a cause of the conflict" and discussed the viability of a transitional government and the need to tackle the humanitarian situation.
The pair also discussed the situation in Yemen and Libya during the Downing Street meeting.