Since late April, the Syrian regime and Russia have stepped up deadly raids on the Idlib region of three million people, a jihadist-held bastion in the country's northwest.

United Nations High Commissioner for Human Rights Michelle Bachelet said she was alarmed at "the apparent international indifference to the rising civilian death toll caused by a succession of airstrikes in Idlib."

Bachelet stressed that medical facilities, schools, markets and other clear civilian targets have been hit.

"These are civilian objects, and it seems highly unlikely, given the persistent pattern of such attacks, that they are all being hit by accident," she said in a statement.

"Intentional attacks against civilians are war crimes, and those who have ordered them or carried them out are criminally responsible for their actions."

More than 730 civilians have been killed in Idlib in air strikes and ground-to-ground fire by the Damascus government and its allies since late April, according to the Syrian Observatory of Human Rights monitoring group.

Syria's opposition has condemned the bombardment as "genocide", while aid groups have branded the carnage in Idlib the latest "nightmare" in the eight-year conflict.

Bachelet said that even as "airstrikes kill and maim significant numbers of civilians several times a week" the international "response seems to be a collective shrug, with the Security Council paralysed."

"This is a failure of leadership by the world's most powerful nations," the rights chief added.

Top UN officials have repeatedly condemned the Security Council's inaction on Syria, with several measures vetoed Damascus ally Russia.

Syria's war has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on protests against President Bashar al-Assad's regime.