John Kerry says US and Russian diplomats have agreed to implement a "cessation of hostilities" in Syria, and to immediately expand the delivery of humanitarian aid.

The US secretary of state, speaking after marathon talks that included Russia and more than a dozen other countries, said that all nations involved in the talks agreed that Syrian peace negotiations should resume in Geneva as soon as possible.

Mr Kerry said that diplomats from the US, Russia, and other powers have agreed to "accelerate and expand" efforts "within a week" to deliver much-needed humanitarian aid to besieged areas immediately.

Foreign Secretary Philip Hammond welcomed the settlement but warned it would succeed only if Russia ceased bombing civilians and moderate opposition groups.

He said:

But some areas most in need of aid will not be included in the deal, because of the presence of the militant Islamic groups Al Nusra and the so-called Islamic State.

A long-term ceasefire in Syria depended on those involved in the bloody civil war engaging in "genuine negotiation", Kerry said.

Speaking after the marathon talks, Russian Foreign Minister Sergei Lavrov said that halting hostilities in the region would be a difficult task.

Syria's main opposition group welcomed the plans, but cautioned that they must see action before further talks in Switzerland could begin.

"If we see action and implementation, we will see you very soon in Geneva," Salim al-Muslat, spokesman for the High Negotiations Committee, told reporters.