Meeting in Korea, US President Barack Obama and Turkish Prime Minister Tayyip Erdogan discussed how to support the opposition with non-lethal aid.
Western and Arab states want Syrian President Bashar al-Assad to stand down but Russia, Syria's long-time ally, said the armed rebels must also cease fire and withdraw their forces.
As Mr Annan, who is the United Nations and Arab League envoy, prepared to meet Russian President Dmitry Medvedev in Moscow, a US-based human rights group accused Mr Assad's forces of using human shields in their efforts to crush the rebellion.
In a statement ahead of the meeting, the Kremlin said it would be hard to enforce a halt to the violence "until external armed and political support of the opposition is terminated".
Mr Annan has drawn up a six-point peace plan, including demands for a ceasefire, the immediate withdrawal of heavy armour from residential areas and access for humanitarian assistance.
But, more than a year after the start of the uprising, the prospect of a negotiated peace seemed more remote than ever, with clashes reported around the country.