Syria's war killed at least 3,000 people including 955 civilians in September, the deadliest month of the conflict this year, the Syrian Observatory for Human Rights monitor said on Sunday.
Hundreds of thousands of Syrians have been killed and millions displaced since the war erupted in 2011 with the brutal repression of anti-government protests.
It has since spiralled into a complex conflict involving world powers, with Russia-backed regime forces and a US-supported alliance separately battling the Islamic State jihadist group in the country.
The 955 civilians killed in September included 207 children, said the Britain-based Observatory, which relies on a wide network of sources inside Syria for its information.
"More than 70 per cent of the civilians were killed in the regime and Russian air strikes, or in air raids of the international coalition" fighting IS, the monitor's head Rami Abdel Rahman said.
Backed by Russian air strikes, the forces of Syria's President Bashar al-Assad are pressing a battle to retake IS-controlled areas in the eastern province of Deir Ezzor.
A US-led international coalition has been providing air support to a Kurdish-Arab alliance, the Syrian Democratic Forces, also fighting the jihadists in their former northern bastion of Raqa city and in Deir Ezzor.
The number of people killed in September was higher because of increased fighting and "intensified air raids of the international coalition and Russia against jihadist bastions in the north and east of Syria, but also due to increased Russian and regime strikes on rebel-held areas," Abdel Rahman said.
Russian and regime warplanes have in the past two weeks increased their strikes on the northwestern province of Idlib, which is largely controlled by Hayat Tahrir al-Sham (HTS), a group led by Al-Qaeda's former Syria affiliate.
Eight children were among at least 34 civilians killed in strikes overnight Friday-Saturday on the town of Armanaz in Idlib, the Observatory said.
HTS is not party to a deal brokered by Russia, Turkey and Iran for a safe zone in the province, one of four such zones nationwide.
The Observatory said the September toll also included 790 regime troops and loyalists, 738 jihadists from IS and HTS, and 550 rebels and SDF members.
Syria's conflict has killed more than 330,000 people since 2011.
Early on Sunday, IS retook the town of Al-Qaryatain in the central province of Homs, previously a symbol of religious coexistence.
Government troops have now surrounded Al-Qaryatain, where several Christian families are believed to be living, the Observatory said.
Regime forces recaptured Al-Qaryatain in April 2016 after eight months of jihadist control.
In August 2015, IS abducted 270 Christians from the town, transporting them around 90 kilometres away deep into the Syria desert and then locking them up in an underground dungeon. They were freed 25 days later.
The same month, IS ravaged a monastery in the town and reduced a fifth-century mud-brick church to rubble with explosives and bulldozers.
Earlier this week, the jihadists launched an assault on government positions in Syria's vast Badiya desert, killing at least 128 regime troops.
Russia-backed Syrian troops have been battling for months to retake the Badiya, which stretches from the country's centre to the Iraqi and Jordanian borders and has been held by IS since 2014.
Last month, they broke a years-long IS siege of government enclaves in the eastern city of Deir Ezzor.