Key players in the Syria crisis have reached agreement on the "key principles" of a way forward towards political transition in the troubled Middle Eastern state, Prime Minister David Cameron has said.

Crucially, Russian President Vladimir Putin, a long-time Syrian ally, had explicitly made clear that he was not "locked in" to Bashar Assad remaining president, said Mr Cameron.

And the PM said that a shipment of attack helicopters that was being tracked in the North Sea had now turned away from Syria, in a success for the international effort to deny Assad access to arms.

As well as Mr Putin, Mr Cameron spoke to the leaders of the US, Germany, France and Turkey about Syria during the two-day G20 summit in Mexico.

Mr Cameron said: "Syria is descending rapidly into a bloody and tragic civil war, with potentially irreparable consequences for its people and the future.

"There is little time left to resolve this, but we do now have clear agreement on the key principles - on the risks to Syria, on the need to stop the violence and the urgency of a political transition from the dreadful position of today to a future where its people can once again make their voices heard and choose their own government."

Mr Cameron added: "There remain differences on sequencing the exact shape of how the transition takes place, but it is welcome that President Putin has been explicit that he is not locked into Assad remaining president in Syria.

"What we need next is an agreement on a transitional leadership which can move Syria towards a democratic future which protects the rights of all its communities. At the same time it makes no sense for any country to supply arms to a regime that is killing its own people with mortars, snipers and attack helicopters.

"We put in place a strong EU arms embargo. We are clearly tracking other shipments to Syria and want to work with countries and companies around the world to stop them, so it is good news that a shipment of attack helicopters that we have been tracking in the North Sea in recent days is now heading away from Syria.

"But we must continue to work to stem the flow of weapons."

The Press Association, photo by Islamic Relief UK