Heavy bombardment by the Syrian army of the extremist-controlled Idlib region has killed 22 civilians, a monitor said Thursday (April 4th), the latest violence to threaten a seven-month-old truce.

The ceasefire was brokered by the main foreign backers of the warring parties in September to head off a government offensive that prompted UN warnings of humanitarian disaster for the region's three million residents, AFP reported.

But since the region was overrun by Tahrir al-Sham alliance in January, the fragile truce has come under mounting assault.

The UN humanitarian affairs office said the escalating violence threatened aid deliveries to some 2.7 million people in need.

In the latest flare-up, army artillery and rocket fire on the Idlib towns of Kafrnabel and Maaret al-Numan killed 13 people on Thursday, the Syrian Observatory for Human Rights said.

It came after shelling of adjacent extremist-held areas of Hama and Aleppo provinces killed nine people late on Wednesday, it said.

The UN humanitarian office said that the escalating violence had already killed 90 civilians in the Idlib region in March, nearly half of them children.

More than 86,500 people fled their homes in February and March as a result of the escalation, it added.

The UN expressed concern over "increased shelling along front lines, an intensification of airstrikes and a growing number of attacks involving improvised explosive devices in urban areas".