David Cameron has promised a "comprehensive strategy" to win MPs' backing for bombing Islamic State militants in Syria as well as Iraq.
The prime minister told the Commons the Paris attacks had strengthened the case for air strikes, suggesting there could be a fresh vote on the issue.
He wanted to "do the right thing for our country", and hit the "head of the snake" of IS in Raqqa, Syria, he said.
MPs rejected possible air strikes in Syria in a 2013 Commons vote.
Labour leader Jeremy Corbyn warned that any military response should have the consent and support of the international community and legality from the United Nations.
The UK has been engaged in coalition air strikes against IS targets in Iraq since being approved by MPs in September last year. Mr Cameron has promised the action will not be extended to Syria without Parliament's consent.
Downing Street has stressed it is not setting out a timetable publicly for a Commons vote yet, says BBC political correspondent Chris Mason.
An "important first step" would be setting out his case in a personal response to a recent report by the Foreign Affairs Committee, the prime minister said, with sources indicating this would be done before the end of the month.
Addressing the Commons on Tuesday, Mr Cameron said that British planes had attacked 350 targets in Iraq and offered support to allies' action over Syria.
But he said Islamic State - also known as Isis, Isil or Daesh - did not recognise or respect the border between the two countries.
"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," he said.
Mr Cameron said: "Raqqa, if you like, is the head of the snake. Over Syria we are supporting our allies - the US, France, Jordan and the Gulf countries - with intelligence, with surveillance and with refuelling. But I believe, as I have said many times before, we should be doing more.
"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."
Cameron's speech came after IS said it was behind Friday's attacks in which 129 people were killed in bars, restaurants, a concert hall and at a stadium in Paris. The victims included Briton Nick Alexander from Essex.