Syrian President Bashar Assad must leave office although the timing of such a move can be negotiated, US Secretary of State John Kerry said in London on Saturday, following talks with British Foreign Secretary Philip Hammond.

Kerry urged Russia and Iran, two key allies, to persuade Assad to negotiate his departure, Reuters said. The top US diplomat also expressed the urgency in finding a political solution to the four-year civil war in Syria, which has left hundreds of thousands dead and millions more displaced.

“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,” Kerry said. “We’re prepared to negotiate. Is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table?”

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Kerry on Friday also stressed the urgency with which he views finding a resolution to the conflict, saying ahead of a meeting with the United Arab Emirates Foreign Minister Abdullah bin Zayed in London that, “this (Syria) crisis has got to be solved.”

American and Russian defense chiefs held their first talks in more than a year on Friday, in a 50-minute conversation that was the first step in direct military talks proposed by Russia about Moscow’s military buildup to support the Syrian government against the Islamic State group.

Meanwhile, after his third phone call with Russian Foreign Minister Sergey Lavrov since Labor Day weekend, Kerry said Friday that Russia proposed talks among the countries’ military leaders “to discuss the issue of precisely what will be done to de-conflict with respect to any potential risks that might be run, and have a complete and clear understanding as to the road ahead and what the intentions are.”

“It is vital to avoid misunderstandings, miscalculations (and) not to put ourselves in a predicament where we are supposing something and the supposition is wrong,” Kerry said.

He said Lavrov had told him Russia was interested only in confronting the threat posed by the Islamic State group in Syria but “I am not taking that at face value.”

Kerry also expressed concern about Russia’s movement of tactical aircraft to Syria — jets that could pose a threat to American and allied forces.

US officials say Russia moved a small number of fighter jets to a base in Syria on Friday, hours after Defense Secretary Ash Carter and his Russian opposite Sergey Shoygu spoke.

AP contributed to this report