The UK-based Syrian Observatory for Human Rights (SOHR) said more than 3,300 people had died in September, including 995 civilians.
Of those civilian deaths, it said about 70% were caused by Russian, Syrian government, or coalition air strikes.
The group bases its casualty reports on information provided by a network of activists in Syria.
It counted 207 children among the civilian dead, along with some 790 pro-government fighters, more than 700 from so-called Islamic State (IS) and al-Qaeda affiliates, and some 550 rebels.
