A Russian official has said for the first time that the Syrian government may be defeated by opposition forces.
President Bashar al-Assad's forces are "losing more and more control and territory", deputy foreign minister Mikhail Bogdanov said.
He said Russia, one of Syria's staunchest allies, was making plans for a possible evacuation of thousands of its citizens.
Separately, Syria denied reports it had fired Scud missiles at rebels.
Nato's Secretary General Anders Fogh Rasmussen earlier said the organisation had detected the launch of such short-range missiles, saying it showed an "utter disregard" for the lives of the Syrian people.