Russia launched an air war on opponents of the Syrian regime of President Bashar al-Assad on September 30 and says it is targeting the Islamic State jihadist group and other "terrorists".
Of the total killed since then, 151 are civilians and include 38 children and 35 women, Observatory chief Rami Abdel Rahman said.
The strikes have also killed 295 combattants, including 75 IS jihadists and 31 members of Al-Qaeda's Syrian affiliate, Al-Nusra Front, Abdel Rahman said.
The Britain-based Observatory says it relies on a network of activists, medical staff and fighters on the ground who identify Russian warplanes based on model, flight patterns and munition types.
Russia has been criticised by rebel groups and their backers for targeting non-jihadist groups and inflicting civilian casualties. It has been also accused of hitting hospitals and field clinics. On Tuesday, the Observatory gave a toll of 370 killed in the Russian air strikes across Syria, a third of them civilians. Moscow has dismissed previous reports by the Observatory of civilian casualties from its bombings as "fake".