Russian air strikes in Syria likely killed more than 1,000 civilians in the war-torn country during the last quarter of 2015, the Airwars monitoring group said Tuesday.

The London-based group gathered media and social media reports -- as well as accounts from rebel groups and non-governmental organizations -- to compile a grim database detailing every known civilian casualty incident that allegedly involved Russian aircraft.

Airwars said its provisional view was that from September 30 -- when Russia entered the Syrian civil war to shore up the regime of President Bashar al-Assad -- and December 31, between 1,096 and 1,448 civilian non-combatants were "likely" to have been killed in 192 Syrian incidents "where Russian strikes appear to have taken place in the near vicinity on that date."

The group also said that 1,700 civilians were "credibly reported injured" in the 192 incidents, an inevitable consequence of Russia's use of non-precision munitions.

"As well as inflicting excessive civilian casualties, Russia is credibly reported to have extensively targeted civilian infrastructure in Syria -- with water treatment plants, bakeries, food distribution depots and aid convoys all struck," Airwars added.

Moscow has denied hitting civilians and says its strikes target "terrorists."

The claimed toll for the three-month period is higher than the total supposedly killed by the US-led coalition that has been hitting Islamic State targets in Iraq and Syria since August 2014.

Airwars put that number at 1,044 but the Pentagon says it is far lower and has so far confirmed the deaths of only 21 civilians, though several investigations into claimed civilian deaths are ongoing.

"Protests by the US and allies at high civilian casualties from Russian airstrikes have been undermined by the coalition's own unwillingness to admit such casualties," Airwars said.

Russian air strikes have tapered off since a partial ceasefire came into force between the Assad regime and rebels.

In all, more than 270,000 Syrians have been killed since the civil war began in 2011, and millions more have fled their homes.

Airwars acknowledged that in some incidents, it was unclear whether the civilians died as a result of Russian bombs or attacks by the regime of President Bashar al-Assad.

Airwars is a collaborative group made up of journalists, researchers and analysts.