US Secretary of State John Kerry said on Saturday Syria's President Bashar al-Assad has to go but the timing of his departure had to be decided through negotiation.

Speaking after talks with British Foreign Secretary Philip Hammond he called on Russia and Iran to use their influence to convince Assad to negotiate.

He said there is an urgency to renewing efforts to find a political settlement to end the four-year war and worsening refugee crisis in Syria.

"We need to get to the negotiation. That is what we're looking for and we hope Russia and Iran, and any other countries with influence, will help to bring about that, because that's what is preventing this crisis from ending," said Kerry.

"We're prepared to negotiate. Is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table?"

He said of Assad's removal: "it doesn't have to be on day one or month one ... there is a process by which all the parties have to come together to reach an understanding of how this can best be achieved."-Reuters