Airstrikes by the Syrian regime and its allies on the northwestern Idlib province have killed at least 100 people, including 26 children, in the past ten days.
The raids have targeted schools, hospitals, markets and bakeries, UN human rights chief Michelle Bachelet said in a statement on Friday.
"These are civilian objects, and it seems highly unlikely, given the persistent pattern of such attacks, that they are all being hit by accident," Bachelet said, adding that the rising toll had been met with "apparent international indifference".
The regime began its offensive against the opposition enclave in northwest Syria, the last area of active insurgent opposition to regime leader Bashar Assad, at the end of April, saying it was responding to violations of a truce.
More than 400,000 people have been displaced over the past three months, said David Swanson of the UN humanitarian affairs agency OCHA on Friday.
Most of the displacement is from southern Idlib and northern Hama, the two areas that have been hit hardest by the flare-up, OCHA said.
"The majority of those fleeing have displaced within Idlib governorate while a smaller number have moved into northern Aleppo governorate.
"Roughly two-thirds of people displaced are staying outside camps," it said.
Since late April, more than 730 civilians have been killed in aerial bombardment and shelling of the region by the regime and its allies, according to Britain-based war monitor, the Syrian Observatory for Human Rights.
OCHA said that since the end of April it had documented 39 attacks against health facilities or medical workers in the region.
At least 50 schools have been damaged by the airstrikes and shelling, it added.
Idlib and surrounding areas of the northwest were included in a "de-escalation" deal last year between Assad's main ally Russia and Turkey, which backs some opposition groups, to reduce warfare and bombardment.
The war in Syria has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-regime protests.
