More than 100 people, including 26 children, have died in air strikes on hospitals, schools, markets and bakeries in north-west Syria in the past 10 days, a top UN official says.
Human rights chief Michelle Bachelet blamed the attacks in rebel-held areas on the government and its allies.But the attacks were met with "apparent international indifference", she said.
Syria and its ally Russia have both denied targeting civilians in air strikes in the Idlib region.
Speaking to reporters, Ms Bachelet criticised the "failure of leadership by the world's most powerful nations".
The rising death toll in Idlib had been met with a "collective shrug" and the conflict had fallen off the international radar, while the UN Security Council was paralysed, she said.
She said the civilian targets were unlikely to have been accidental and warned that those carrying out the attacks could be charged with war crimes.
What is happening in Syria?Idlib province, along with the north of Hama province and western Aleppo province, is one of the last opposition strongholds in Syria after eight years of civil war.
It is supposedly covered by a truce brokered in September by Russia and opposition-backer Turkey that spared the 2.7 million civilians living there from a major government offensive.