Russia's prime minister has said Syrian President Bashar al-Assad made a "grave, perhaps fatal error" by delaying political reforms, as Damascus courted opposition forces.

"He should have acted much more quickly and reached out to the peaceful opposition which was ready to sit at the negotiating table with him," Russian news agencies quoted Dmitri Medvedev as saying on Sunday.

"It's a grave error on his part, perhaps fatal," Medvedev said.