Syria conflict: Kerry and Hammond seek negotiations
US Secretary of State John Kerry has called for a renewed diplomatic effort to end the conflict in Syria.
Speaking after talks in London, he said what he described as Russia's new focus on fighting Islamic State militants could be an opportunity to push towards a political settlement.
He said this was all the more urgent as violence in Syria was the "root cause" of Europe's current migrant crisis.
The EU, which is divided on the issue, is to hold emergency talks next week.
Mr Kerry is on a European tour which is expected to be dominated by the Syrian conflict.
After talks with his UK counterpart Philip Hammond on Saturday, he said they "agreed completely on the urgency of nations coming together in order to resolve this war that has gone on for much too long".
Analysis - Sebastian Usher, BBC Middle East analyst
It's been a long time since there's been serious talk of working towards a diplomatic solution to the Syrian conflict. The refugee crisis and the rise of IS have provided the spur.
Just as importantly, perhaps, a series of defeats for Syrian President Bashar al-Assad has made his position look more precarious than it has for years, possibly enough to shift the stalemate and bring the Syrian government to genuine negotiations.
For now at least, it seems to have prompted Russia to start bolstering its military support for Mr Assad in ways that have worried the US.
The question for the West is whether Russia wants to help the Syrian government fight IS or simply keep Mr Assad in power. Either way, it's making the Syrian equation even more complicated.
He added that the migrant crisis had to "be addressed by dealing with the root cause" - which he said was violence in Syria.
For more than four years the government of President Assad has been fighting various rebels groups - including IS, which controls vast swathes of territory in both Syria and neighbouring Iraq.
Mr Kerry said there was a need to "use this moment when Russia appears more committed to doing more" against IS, to move towards a diplomatic solution.
"We're prepared to negotiate. Is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table?"
Ahead of the meeting he had said the US shared the "same goals" with Russia - a long-time ally of President Assad - in the battle against IS.
The US has called for Mr Assad to go, but on Saturday Mr Kerry reaffirmed the US position that this removal "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."
Mr Hammond said the Syrian president could not be part of Syria's long-term future "but the modality and timing has to be part of a political solution".
Mr Kerry said the London talks laid the groundwork for forthcoming meetings at the United Nations over Syria.
Russian intentions may become clearer when President Vladimir Putin addresses the UN General Assembly later this month, says the BBC's Sebastian Usher.
More than 200,000 Syrians have been killed since the conflict began in 2011. More than four million have fled abroad and many more are internally displaced.
The conflict has contributed to the migrant crisis in the EU - which is facing a huge influx this year.
UK Prime Minister David Cameron has announced that the UK will accept up to 20,000 refugees from camps surrounding the country.
Mr Kerry is due to go to Berlin on Sunday.