David Cameron has signalled he will ask MPs to approve Britain launching air strikes on Isis targets in Syria by Christmas.
He promised to set out a "comprehensive strategey" for defeating Isis and said the case for military intervention in Syria had "only grown stronger" since Islamic terrorists killed 129 people in the devestating terror attacks in Paris on Friday.
But he will only go ahead with a vote in the House of Commons if he is confident of winning, Downing Street said.
Ministers will set out the plans to bomb Isis in Syria by the end of the month, meaning the first air strikes could be launched before Christmas.
In a Commons statement following the G20 summit of world leaders in Turkey, Mr Cameron told MPs: "We must ask ourselves if we really are doing all we can be doing, all we should be doing, to deal with the threat of ISIL and the threat is poses to us directly."
He and other ministers have described as "absurd" the current situation where Britain is carrying out airstrikes on Isis in Iraq but not over the border in Syria.
However he insisted he understood concerns about bombing in Syria but added: "I know they must be answered; I know they can be answered".
The Prime Minister told MPs he will take the rare move of responding personally to the Foreign Affairs Select Committee report that set out grave reservations about expanding the RAF bombing campaign to Syria.
He signalled he would then seek to win a vote backing military intervention in the war-torn country.
The BBC has reported that the Ministry of Defence will hold briefings with MPs to set out the case for military action in the first week of December.
He added that Britain must share the "burden" in the international fight against Isis and said the UK must do more by targeting Raqqa, the de-facto capital of Isis in Syria, which Mr Cameron described as the "head of the snake".
Jeremy Corbyn said he "stands ready" to work with the Government in its fight against Isis but said any plans to go ahead with military action must have legal backing and support from the United Nations.
However, answering questions from MPs later, Mr Cameron disputed the need for UN backing, saying: "We cannot outsource our national security to a Russian veto."
