Violence in the northwestern Syrian region of Idlib has displaced more than 150,000 people in the past week, the UN said Tuesday, as the regime and Russia upped deadly bombardment of the militant bastion.

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 militant 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 militant Hayat Tahrir Al Sham group took full control of it in January.

On Tuesday, air strikes and shelling killed 13 civilians in an eighth day straight of bombing, the Syrian Observatory for Human Rights said.

Syrian Civil Defence members help a man carrying a victim after a deadly airstrike in Ras El Ain, Idlib.

"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 the media.

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.

'Urgent de-escalation'

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 per cent of the country's eight years into the civil war, but Idlib is among the areas still outside government control.

Battles between militants 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 militants were killed in fierce fighting, the Britain-based Observatory said. The militants were members of HTS, and of the Turkistan Islamic Party, a Uighur-dominated militant 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.

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.

'Limited offensive'

To the west of the region on Tuesday, the Al Qaeda-linked Hurras Al Deen militant group attacked pro-government positions, killing nine loyalists and three militants, the Observatory said.

It remains unclear whether the Syrian government and its Russian ally are planning to launch a full-scale assault.

Aron Lund, from the US think tank The Century Foundation, said "a limited offensive into Idlib, peeling off a few areas, should be easily within their capabilities".

He said the recapture of two key highways running through Idlib -- the M4 and the M5 — could be among the "many goals" behind such an operation.

Under the September deal, hardliners were supposed to withdraw from the planned buffer zone, allowing traffic to once again flow along the two strategic highways, which connect government-held areas with the Turkish border.

Turkey has failed, however, to secure the militants' withdrawal, prompting government forces to take matters into their own hands, Syria specialist Fabrice Balanche said.

Taking the two highways would help Assad boost the recovery of Syria's nearby second city Aleppo, which remains cut off from most of its countryside and poorly connected to the rest of the country, he told AFP.

"Restoring traffic on these two axes will reduce transport costs to Aleppo," he said.

Retaking the road between the regime's coastal stronghold of Latakia and Aleppo in particular would cut the rebel-held region in two, making it easier for government forces to recapture its southern part and isolate the militants in the north, Balanche added.

Agence France-Presse