“More than 400,000 people have been displaced since the end of April,” said David Swanson of the United Nations humanitarian affairs agency OCHA.
The region under attack is home to some three million people, nearly half of them already displaced from other parts of the country.
It covers nearly all of Idlib and parts of neighbouring Aleppo, Hama, and Latakia provinces.
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.
The region is controlled by jihadist alliance Hayat Tahrir al-Sham, led by Al-Qaeda’s former Syria affiliate.
Since late April, more than 730 civilians have been killed in aerial bombardment and shelling of the region by the Syrian government 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.
A September accord struck between Moscow and Ankara was supposed to spare the region the bloodshed of a government assault, but it was never fully implemented after jihadists refused to withdraw from a planned buffer zone.
Instead, the bombardment has increased in intensity in recent weeks.
Airstrikes by the government and its Russian ally killed 12 civilians in the region on Thursday, according to the Observatory.
Another 50 civilians were killed in strikes on Monday alone – the majority on a busy market.
OCHA described Monday as one of the “deadliest days” in the region since the start of the flare-up.
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-government protests.