London, UK (BBN)-David Cameron says air strikes against Islamic State militants in Syria would be in the UK's "national interest".
The prime minister denied claims it would make the UK a bigger target for terror attacks, as he made the case for military action, in the Commons, reports BBC.
He told MPs the UK was already a target for IS - and the only way to deal with that was to "take action" now.
The UK could not "outsource our security to allies" and it had to stand by France, he added.
A Commons vote on authorising air strikes is expected within weeks.
Labour leader Jeremy Corbyn sought assurances the UK would not be dragged into a ground war and asked whether UK air strikes would make any military difference.
He said there was "no doubt" the "so-called Islamic State group has imposed a reign of terror on millions in Iraq, Syria and Libya" and that it "poses a threat to our own people".
'MORALLY UNACCEPTABLE'
But he added: "The question must now be whether extending the UK bombing from Iraq to Syria is likely to reduce, or increase, that threat and whether it will counter, or spread, the terror campaign Isis is waging in the Middle East."
Cameron told MPs the UK could not afford to "stand aside" from the fight and it was "morally" unacceptable to "outsource our security" to allies such as France and America.
He said "we face a fundamental threat to our security" and could not wait for a political solution, and that doing nothing "could make the UK more of a target for Isil attacks".
He ruled out British "boots on the ground" - and insisted the RAF had specialist bombing capabilities that were needed by its allies.
He also addressed concerns about the downing of a Russian jet, insisting procedures were in place to reduce the risk of a similar incident.
'COMPREHENSIVE STRATEGY'
He insisted there was strong legal justification for extending the current military action in Iraq, on grounds of self-defence and the recent UN Security Council resolution.
He stressed that IS could not be defeated by air strikes alone, but they were was a key part of a wider "comprehensive" strategy to deal with the threat.
Summing up his argument for air strikes, he asked MPs: "If not now, when?"
He added that there would not be a vote in the Commons unless there was a majority for action "because we will not hand a publicity coup to Isil".
The prime minister's case is set out in detail in his earlier response to a recent Foreign Affairs Committee report setting out the tests for military intervention.
In it, he calls for IS to be denied a "safe haven" in Syria and says it is wrong for the UK to "expect the aircrews of other nations to carry the burdens and the risks of striking Isil in Syria to stop terrorism here in Britain".
The government's strategy will include:
providing humanitarian support to Syrians
planning for the "reconstruction" of war-torn Syria
working with international partners to tackle IS
Cameron, whose statement comes just under a fortnight since the attacks in Paris that killed 130 people, will need to convince enough MPs from other parties to back his case in order to offset any Conservative rebels.
Corbyn is under pressure to offer his MPs a free vote but is hoping his shadow cabinet can come to a "collective view" on the issue.
MPs rejected strikes against Syrian government forces in a 2013 vote, but have since authorised action against IS in Iraq.
The SNP, which has 54 Westminster MPs, has said it will not back military intervention without a specific authorisation from the United Nations.
The Democratic Unionist Party, which has eight MPs, has suggested it could be open to backing air strikes - in 2013 five of its MPs voted against the government.
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