Observatory for Human Rights said on Sunday.
Over 70 per cent of the civilians were killed in airstrikes, the monitor’s head Rami Abdel Rahman said.
The forces of Syrian President Bashar al-Assad are pressing a battle to retake Islamic State-controlled areas in the eastern province of Deir al-Zour, Xinhua news agency reported.
A US-led coalition has been providing air support to a Kurdish-Arab alliance also fighting the jihadists in its former northern bastion of Raqqa city and Deir al-Zour.
The number of people killed in September was higher due to increased fighting and “intensified air raids against jihadist bastions in the north and east of Syria”, Abdel Rahman said.
The 955 civilians killed in September included 207 children, said the observatory, which relies on a wide network of sources inside Syria for its information.
Syria’s conflict has killed more than 330,000 people and displaced millions since it broke out in 2011.