More than 150,000 displaced in northwest Syria
The uptick in strikes and shelling on the region dominated by Syria's former al-Qaeda affiliate has also knocked 12 hospitals and 10 schools out of action, it said.
The jihadist stronghold has since September been protected from a massive regime offensive by a buffer zone deal inked by Damascus ally Russia and rebel backer Turkey.
But the region of some three million people has come under increasing bombardment since the jihadist Hayat Tahrir al-Sham group took full control of it in January.
On Tuesday, airstrikes and shelling killed 13 civilians in an eighth day straight of bombing, the Syrian Observatory for Human Rights said.
"We are alarmed by ongoing reports of aerial attacks on population centres and civilian infrastructure," said David Swanson, a spokesman for the UN Office for the Coordination of Humanitarian Affairs (OCHA).
"More than 152,000 women, children and men have been displaced in Aleppo and Idlib governorates over the past week alone," he told AFP.
The recent surge in attacks has raised new fears a government offensive is imminent, prompting thousands to hit the road.
"This is the third time we have been displaced but this time is the scariest," said Abu Ahmad, a 40-year-old from southern Idlib who was fleeing Tuesday with his family towards areas near the border with Turkey.
"Overflights by warplanes and shelling have been relentless," said the father of three, his blue pick-up truck stacked with mattresses and household appliances.
The Idlib region includes a large part of the province of the same name, as well as adjacent parts of Aleppo and Hama provinces.
President Bashar al-Assad's regime is in control of around 60 percent of the country eight years into the civil war, but Idlib is among the areas still outside government control.
Battles between jihadists and pro-government forces raged overnight around a hilltop in the northern countryside of Hama province, following an advance by Assad's forces.
Twenty-four pro-government fighters and 29 jihadists were killed in fierce fighting, the Britain-based Observatory said. The jihadists were members of HTS, and of the Turkistan Islamic Party, a Uighur-dominated jihadist group.
State news agency SANA said Syrian troops launched rocket attacks on armed groups in northwestern Hama province on Tuesday, killing several fighters, but it did not provide any toll.
Since 28 April, at least seven healthcare facilities have reportedly been struck, including four in Hama and three in Idlib, said David Swanson of the UN Office for the Coordination of Humanitarian Affairs.
Russia and rebel-backer Turkey in September inked a buffer zone deal to prevent a massive regime offensive on the Idlib region, near the Turkish border.
But the region of some three million people has come under increasing bombardment since HTS took full control of it in January.
UN chief Antonio Guterres has called "for an urgent de-escalation of the situation as the holy month of Ramadan begins" and urged "the parties to recommit fully to the ceasefire arrangements" of the September deal.
French President Emmanuel Macron on Twitter demanded "a halt to the violence and support to the UN in backing a necessary political solution".
OCHA said that bombardment on the Idlib region since April 28 had also killed three health workers.
Agencies contributed to this report.
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