British Foreign Secretary William Hague said on Saturday he was uncertain that world powers could reach a power transition deal to halt the bloodshed in Syria, as Russia and China remain opposed.

"We haven't reached agreement in advance with Russia and China. That remains very difficult and whether it will be possible, I don't know if this will be possible," he told reporters on arriving at the United Nations in Geneva for talks on the crisis hosted by international envoy Kofi Annan.