The UK cannot afford to wait for a political settlement in Syria before targeting Islamic State in the country, David Cameron has said.

The Prime Minister told MPs "we should be acting now" to take the fight to the terror group in Syria.

His statement came after he was asked during Prime Minister's Questions how many troops he believes will be needed to defeat Islamic State, also called Daesh and Isil, on the ground.

Angus Robertson, the SNP's leader in Westminster, asked Mr Cameron: "The fatal dangers of unintended consequences and escalation in Syria are clear for everybody to see in these days.

"All serious observers agree that an air campaign alone will not lead to the ultimate defeat of Daesh on the ground and that ground forces will be needed.

"How many troops and from which countries does the Prime Minister have in his plan for Syria?"

Mr Cameron said: "I am not for one minute arguing that action from the air alone can solve the very serious problem we have with Isil.

"Clearly we need a political settlement in Syria and a government in Syria that can act comprehensively with us against Isil.

"The question for the House which we need to address tomorrow and in the days to come is: Should we wait for that?

"Can we afford to wait for that political settlement before we act? And my view is no, we can't wait for that political settlement.

"We should work as hard as we can for it but we should be acting now with allies because it is about keeping our own people and our own country safe."

On the issue of troops, Mr Cameron said there are already forces in Syria in the shape of the Free Syrian Army and Kurdish units which "would work with us to help eliminate Isil".

"But of course the full range of ground troops will only be available when there is a political settlement in Syria," he said.

"But the question is simple: Can we afford to wait for that political settlement before taking action to keep us safe here at home and my answer to that is no, we can't afford to wait."

Mr Robertson also asked how much money the Government will provide to help rebuild Syria.

Mr Cameron said funds will be forthcoming.

"I would far rather, frankly, spend the money reconstructing Syria than in supporting people kept away from their homes, kept away from their country who dearly want to return," he said.