Prime Minister David Cameron says Britain must join airstrikes in Syria to deny the Islamic State group a "safe haven" from which to plot mass-casualty attacks around the world.

Cameron is trying to persuade lawmakers to back action, arguing that the Paris attacks have given new urgency to the fight against IS.

The Royal Air Force is part of a U.S.-led coalition attacking the militants in Iraq, but not in Syria.

Earlier this month Parliament's Foreign Affairs Select Committee said British airstrikes would be "incoherent" and ineffective without a plan to end Syria's civil war.

Cameron replied Thursday in writing and is due to make his case in the House of Commons for airstrikes as part of a "comprehensive overall strategy" to destroy IS and end the Syrian war.