US Secretary of State John Kerry says Syrian President Bashar al-Assad "has to go," but the top US diplomat said the timing of Assad's departure should be decided through negotiation.

Speaking on Saturday after talks with British Foreign Secretary Philip Hammond in London, Kerry called on Russia and Iran to use their influence over Assad to convince him to negotiate a political transition.

Kerry said the US welcomed Russia's involvement in tackling the so-called Islamic State in Syria — the Syrian military has started using new types of air and ground weapons supplied by Russia in recent days — but a worsening refugee crisis underscored the need to find a compromise that could lead to political change in the country.

"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," he added. "Is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table?"

Russia's buildup at Syria's Latakia airbase has raised the possibility of air combat missions in Syrian airspace. Heavy Russian equipment, including tanks, helicopters and naval infantry forces, have been moved to Latakia, US officials say.

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

"For the last year and a half we have said Assad has to go, but how long and what the modality is… that's a decision that has to be made in the context of the Geneva process and negotiation," Kerry said.

"It doesn't have to be on day one or month one," he added. "There is a process by which all the parties have to come together and reach an understanding of how this can best be achieved."

Kerry said he did not have a specific time frame in mind for Assad to stay in power. "I just know that the people of Syria have already spoken with their feet. They're leaving Syria."

Kerry also met in London with United Arab Emirates Foreign Minister Abdullah bin Zayed, who said his country has taken in more than 100,000 refugees in the last four years, "A huge burden for a very small country."

Hammond, who on September 9 said Britain could accept Assad staying in place for a transition period, said the ruler could not be part of Syria's long-term future "but the modality and timing has to be part of a political solution that allows us to move forward."

Assad's forces have repeatedly been accused of dropping crude barrel bombs on residential neighborhoods, indiscriminately killing thousands of civilians. Amnesty International reported that approximately 3,000 people were killed in 2014 by barrel bombs in Syria's Aleppo province. Assad has also been accused of using chlorine gas in attacks, though the regime has denied both allegations.

Hammond said the situation in Syria was now more complicated with Russia's increased military involvement in the country.

"Because of the Russian engagement the situation in Syria is becoming more complicated and we need to discuss this as part of a much bigger problem — the migration pressures, the humanitarian crisis in Syria as well as the need to defeat ISIL," he said, using an alternate acronym for the Islamic State.

Kerry and Hammond said they also discussed conflicts in Yemen, Libya and Ukraine.

The Pentagon said on Friday that Defense Secretary Ash Carter had talked to his Russian counterpart, Sergei Shoigu, the first conversation between the two since Carter took over at the Department of Defense seven months ago. Earlier in the week, the Pentagon said Carter had no plans to talk to the Russians.

"Right now, Secretary Kerry is taking the lead with regard to discussions with Russia. And we're going to leave it at that," Pentagon spokesman Peter Cook told a news briefing, according to Reuters.

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