UN investigators said that "indiscriminate bombardment" by pro-regime forces ahead of a March ceasefire brokered with Turkey, claimed hundreds of lives.
Syrian and Russian planes have carried out deadly aerial strikes amounting to war crimes on schools, hospitals and markets in Idlib province.
UN investigators said in a report on Tuesday that "indiscriminate bombardment" by pro-regime forces, ahead of a March ceasefire brokered with Turkey, claimed hundreds of lives and forced nearly one million civilians to flee, which may amount to a crime against humanity.
The UN Commission of Inquiry on Syria also accused Hayat Tahrir al Sham (HTS), a former Al Qaeda affiliate that controls part of northwest Syria, of firing artillery into civilian areas "with no apparent legitimate military objective".
Fighters from HTS have tortured and executed detainees, it added.
"What is clear from the military campaign is that pro-government forces and UN-designated terrorists flagrantly violated the laws of war and the rights of Syrian civilians," Paulo Pinheiro, chairman of the UN panel, said in a statement.
The report, covering November 2019 until June 2020, was based on overflight data and witness testimony.
It examines 52 "emblematic attacks" in northwest Syria, including 47 attributed to the Russian-backed Syrian regime.
Russian warplanes were solely implicated in a deadly March 5 strike on a poultry farm near Marat Misrin that sheltered displaced people and in three strikes next to a hospital damaged in the rebel-held town of Ariha on January 29, the report said. Russia denies involvement in the latter attack, it said.
"The commission has reasonable grounds to believe that pro-government forces committed the war crimes of deliberately attacking medical personnel and facilities by conducting air strikes," it said.
Karen Koning AbuZayd, a panel member, said: "women, men and children that we interviewed faced the ghastly choice of being bombarded or fleeing deeper into HTS-controlled areas where there are rampant abuses of human rights ..."
"The acts by HTS members amount to war crimes."