Russia has offered "full support" for international efforts to end a year of fighting in Syria but gave no details of how that would play out on the ground as forces loyal to President Bashar al-Assad attacked Homs and other rebel strongholds.
Moscow reiterated its stance that foreign support for the Syrian opposition was the main obstacle to peace.
Western and Arab states have urged Syrian President Bashar al-Assad to step aside to end violence which the UN says has cost 8,000 lives.
Russia, a close ally of Mr Assad, said if he is to end his assault, the rebels must also cease fire.
Russian President Dmitry Medvedev, who will join Obama for a nuclear security summit in South Korea on Monday, told UN-Arab League Syria envoy Kofi Annan in Moscow he appreciated his efforts to end the violence.
"This may be the last chance for Syria to avoid a long-lasting and bloody civil war. Therefore we will offer you our full support at any level and in various ways in those areas, of course, in which Russia is capable of providing support."
It was not clear whether Moscow would use its influence with Mr Assad to press him to end his assault on his opponents and allow access to humanitarian aid.
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.
"Syria has an opportunity today to work with me and this mediation process to put an end to the conflict, to the fighting, allow access to those in need of humanitarian assistance as well as embark on a political process that will lead to a peaceful settlement," Mr Annan's statement said.
West is 'too one-sided'
Moscow has accused the West of being too one-sided, arguing that outside support for rebels, which it says is both political and military in some cases, is fuelling the fighting in Syria, which hosts a Russian naval base.
Mr Assad, it argues, is ready for talks
Russia and China have vetoed two UN resolutions critical of Damascus, but did support a Security Council statement this week endorsing Annan's mission. The former UN chief is due to fly to China after his Russia talks.
