LONDON (Reuters) - U.S. Secretary of State John Kerry said on Saturday Syria's President Bashar al-Assad has to go but the timing of his departure should be decided through negotiation.

Speaking 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 United States welcomed Russia's involvement in tackling the Islamic State in Syria but a worsening refugee crisis underscored the need to find a compromise that could also 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," said Kerry.

"We're prepared to negotiate. 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, U.S. officials say.

Kerry said of Assad's removal: "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 added: "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 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. "I just know that the people of Syria have already spoken with their feet. They're leaving Syria."

Hammond, who on Sept. 9 said Britain could accept Assad staying in place for a transition period, said Assad 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."

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.

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

(Writing by James Davey, editing by Clelia Oziel)