US Secretary of State John Kerry has said Syria's President Bashar al-Assad has to go but the timing of his departure should be decided through negotiations.
Speaking after talks with British Foreign Secretary Philip Hammond in London on Saturday, 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 of Iraq and the Levant (ISIL) group in Syria, but a worsening refugee crisis underscored the need to find a compromise that could also lead to political change in the country.
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"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 build-up 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 said.
Kerry said that Assad's removal has to be made in the context of the Geneva process and negotiation.
"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 timeframe in mind for Assad to stay.
"I just know that the people of Syria have already spoken with their feet. They're leaving Syria."
Al Jazeera's Nadim Baba, reporting from London, said that the demand for Assad to leave remains the key sticking-point in the on and off talks to end the conflict in Syria.
"Nonetheless it is clear that the secretary wants to speed up things in Syria politically and militarily," he said.
Hammond, who on September 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.
Source: Al Jazeera and agencies