Heavy bombardment by the Syrian army of the jihadist-controlled Idlib region has killed 22 civilians, a monitor said Thursday, the latest violence to theaten 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.
But since the region was overrun by the Hayat Tahrir al-Sham alliance led by former Al-Qaeda fighters in January, the fragile truce has come under mounting assault.
Syrian Foreign Minister Walid Muallem accused Turkey, which signed the September deal on behalf of the rebels, of failing to honour its commitments and warned that his government's patience was running out.
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-Noman killed 13 people on Thursday, the Syrian Observatory for Human Rights said.
It came after shelling of adjacent jihadist-held areas of Hama and Aleppo provinces killed nine people late on Wednesday, the Britain-based watchdog said.
The UN humanitarian office said that the ecalating 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 air strikes and a growing number of attacks involving improvised explosive devices in urban areas."
The Syrian foreign minister said his government was growing impatient to recapture Idlib, the last region outside its control apart from the Kurdish-held north and northeast where Washington retains a troop presence.
Muallem said Turkey had failed to ensure the withdrawal of jihadist forces from a planned buffer zone along the front line as stipulated by the truce agreed in the Russian Black Sea resort of Sochi.
"It is known that Turkey is responsible for a delay in implementing" the deal, he said at a joint press conference with his Venezuelan counterpart Jorge Arreaza.
"Honestly, we are still waiting for the Sochi deal to be implemented but our patience has its limits and we must liberate this land.
"We are losing patience," he warned.
More than half of the population of the Idlib region have already fled government offensives on other rebel-held regions of Syria.
Many live in tent cities where they are dependent on humanitarian aid and deeply vulnerable to a resurgence of all-out conflict.
At least 370,000 people have died in Syria since the civil war erupted in 2011.