Amnesty International on Monday said it has documented 18 attacks in northwest Syria carried out by government and Russian forces over the past year that amounted to "war crimes".
Russia-backed forces have, since April 2019, waged two deadly military campaigns against Syria's last major rebel bastion of Idlib, home to about three million people.
A ceasefire has largely held since March, but hundreds of thousands remain displaced and dependent on aid even as the militant-dominated region braces for a possible outbreak of the novel coronavirus.
Amnesty said it had documented 18 attacks on medical facilities and schools by either Damascus or its Russian ally between May 5, 2019, and February 25 in and adjoining the rebel stronghold.
"Evidence shows that, in their entirety, the documented attacks by Syrian and Russian government forces entailed a myriad of serious violations of international humanitarian law," it said.
"These violations amount to war crimes."
Amnesty said they included three ground attacks and two barrel bomb attacks by Syrian government forces, as well as air strikes by both or either force.
It said the majority occurred in January and February 2020, during the latest onslaught which from December has killed around 500 civilians and displaced almost a million people.
Among the documented attacks were Russian air strikes near a hospital in the town of Ariha on January 29 that flattened at least two residential buildings and killed 11 civilians, it said.
Amnesty also blamed the Syrian government for an attack using internationally banned cluster munitions that killed three people in a school in Idlib city on February 25.
"The latest offensive continued an abhorrent pattern of widespread and systematic attacks aimed at terrorising the civilian population," Amnesty's regional director Heba Morayef said.
"Russia has continued to provide invaluable military support – including by directly carrying out unlawful air strikes – despite evidence that it is facilitating the Syrian military's commission of war crimes and crimes against humanity."
Syria's war has killed more than 380,000 people and displaced millions since it started in 2011 with the repression of anti-government protests.