War crimes and possible crimes against humanity were committed during the battle for Syria's opposition-held Idlib province, a UN investigation said on Tuesday.

The Independent International Commission of Inquiry on Syria said people endured "unfathomable suffering" during the campaign launched in late 2019 by pro-regime forces to retake the last remaining areas in the country held by armed groups.

"Children were shelled at school, parents were shelled at the market, patients were shelled at the hospital, and entire families were bombarded even while fleeing," said commission chair Paulo Pinheiro.

Syrian President Bashar al-Assad's regime, supported by Russia, in December relaunched its offensive against the northwestern region, which is dominated by the Hayat Tahrir al-Sham (HTS) jihadist group.

A precarious truce brokered by Russia and rebel-backer Turkey came into force in early March.

The offensive left one million people displaced and more than 500 civilians dead, according to the United Nations.

The 29-page commission report covers the period from November 1 to June 1.

It documents 52 attacks -- by all parties -- that led to civilian casualties or damage to civilian infrastructure.

They included 17 attacks affecting hospitals and medical facilities, 14 involving schools, nine on markets, and 12 others on homes.

These attacks were "marked by war crimes", the report said.

"They foreseeably led to massive displacement, as civilians had no choice but to flee, and may amount to crimes against humanity," the report said.

The report accused both government forces and HTS of pillaging and other war crimes.

'Completely abhorrent' 

Hanny Megally, one of the three people on the commission, said war crimes were likely to have been committed by both the Syrian and Russian air forces.

"We document two incidents where we think it was Russian aeroplanes that conducted those attacks," he told a virtual press conference.

Pinheiro said there was no explanation as to why such predictable levels of suffering were permitted to proceed.

"It is completely abhorrent that, after more than nine years, civilians continue to be indiscriminately attacked, or even targeted, while going about their daily lives," he said.

"Pro-government forces and UN-designated terrorists flagrantly violated the laws of war and the rights of Syrian civilians."

The Brazilian legal scholar told the press conference he feared having to return in future to report that "this bloodshed continues to go unchecked".

The commission urged all parties to the conflict in Syria to cease attacks on civilians, and also urged countries to pursue accountability for the documented crimes.

The report is due to be presented at the Human Rights Council on July 14-15.

The war in Syria has killed more than 380,000 people and displaced nearly half of the country's pre-war population since it started in 2011.