US Secretary of State John Kerry has called for a renewed diplomatic effort to end the conflict in Syria.
Mr Kerry was speaking after talks in London, and said 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.
Meanwhile, volunteer military troops have reportedly been called up in Hungary to deal with the number of refugees arriving in the country.
The news comes as a the Greek coastguard says a five-year-old girl has died and up to 13 migrants are missing off the island of Lesbos.
