Amnesty International on Monday said it has documented 18 attacks in northwest Syria carried out by regime and Russian forces over the past year that amounted to "war crimes".

Russia-backed regime forces have since late April 2019 waged two deadly military campaigns against Syria's last major rebel bastion of Idlib, which has become home for some three million people.

A ceasefire has largely held since early March, but hundreds of thousands remain displaced and highly dependent on aid even as the jihadist-dominated region braces for a possible outbreak of the novel coronavirus.

The rights group said it documented 18 attacks on medical facilities and schools by either Damascus or its Russian ally between 5 May 2019 and 25 February 2020 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 side. 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 regime for an attack on a school using internationally banned cluster munitions that killed three people 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 brutal repression of anti-government protests.