US Secretary of State John Kerry has called on Russia and Iran to bring Syria's Bashar al Assad to the negotiating table.
Mr Kerry spoke after talks in London with British Foreign Secretary Philip Hammond over the Syria war, which has sparked a migration crisis across Europe.
The top US diplomat said America was prepared to negotiate.
"We need to get to the negotiation," he said.
"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.
"We're prepared to negotiate. Is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table?"
He said Mr Assadneeds to be removed, but added that "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 to reach an understanding of how this can best be achieved."
The London talks came shortly after US officials saidRussia had sent at least four fighter jets to Syriato bolster Mr Assad's embattled forces.
Mr Kerry said the US is disturbed by Russia's movement of the tactical aircraft.
Islamic State extremists have taken over parts of Syria and Iraq.
Mr Hammond said: "Because of the Russian engagement, the situation in Syria is becoming more complicated.
"I think 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."
Meanwhile, the first talks between US and Russian defence chiefs in more than a year have taken place.
US Defence Secretary Ash Carter and his Russian counterpart, Sergei Shoigu, spoke on the phone for 50 minutes.
They discussed ways to avoid accidental military interactions in Syria's limited air space, where the US and its allies are carrying airstrikes against Islamic State.
Ahead of the meeting in London, Mr Kerry said Russia and America "share the same goals" in the battle against IS extremists.
By sky News
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