Prime Minister David Cameron told MPs on Thursday it was time to join air strikes against ISIS militants in Syria, saying Britain cannot "subcontract its security to other countries".

Cameron, who lost a vote on air strikes against Syrian President Bashar al-Assad's forces in 2013, needs to persuade several MPs in his own Conservative Party and some in the opposition Labour Party to back his cause if he is to win parliament's backing for military action.

Parliament's Foreign Affairs Committee criticised extending air strikes into Syria earlier this month, saying that without a clear strategy to defeat the militants and end the civil war such action was "incoherent".

But since ISIS claimed responsibility for killing 130 people in Paris, some MPs who were reluctant to launch new strikes in Syria have increasingly felt action was needed to protect Britain from such attacks.

"We do not have the luxury of being able to wait until the Syrian conflict is resolved before tackling ISIL," Cameron wrote in a response to the committee's objections.

He said in his 24-page response the campaign against ISIS was entering a new phase, focussing on command and control, supply lines and financial support - something Britain could contribute well to.

"It is wrong for the United Kingdom to sub-contract its security to other countries, and 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," he said.

Cameron will have to convince MPs that extending the air strikes will not mean that Britain becomes an even greater target for attacks. Militants downed a Russian airliner after Moscow launched strikes on Syria, killing all 224 on board.

Thirty Conservative MPs voted against the motion for military intervention in Syria in August 2013.

Labour leader Jeremy Corbyn, a veteran anti-war campaigner, says he is also reluctant to support the strikes without a political plan for Syria, fearful more bombing would complicate the more than 4-1/2 year civil war.

But, breaking with a British political tradition of using a "party whip" to keep party discipline, his finance spokesman said Labour was considering allowing its MPs to vote as they wish, which may increase support for Cameron.

"In these sort of issues of conscience it is better to allow MPs to make their own minds up," John McDonnell told BBC television, adding Britain must learn lessons from the 2003 Iraq war. ISIS hold territory in northern and western Iraq.

Finance minister George Osborne, a frontrunner to succeed Cameron, said he understood people's concerns that further involvement in the Middle East could make Britain a target.

"We know they want reassurance that we are getting this right ... first of all we are a target from this terrorist organisation ISIL," he told BBC radio, referring to the attacks on tourists in Tunisia in June.

He said it was a "bit strange" that Britain's air force could strike in Iraq but not Syria.

"I don't think this is a country that lets others like the French or the Americans defend our interests and protect us from terrorist organisations - we should contribute to that effort."

By Reuters