BEIRUT: At least 15 people were reported killed on Sunday in shelling by regime and insurgent forces in northwestern Syria, further straining a Russian-Turkish cease-fire deal for the region.

The northwest is the last major foothold of fighters opposed to Syria’s Bashar Assad, many of whom were forced to retreat there after military defeats at the hands of regime forces backed by Iran and Russia.

Last year, Damascus was poised to mount an offensive into the northwest, raising fears of a humanitarian catastrophe. The assault was postponed after Moscow struck a deal with Ankara that included the creation of a “demilitarized zone.”

But the agreement has come under strain in recent weeks: The Syrian Observatory for Human Rights said 45 people had been killed in the last five days alone, most of them by regime shelling of opposition-held areas.

On Sunday, regime shelling killed seven people in opposition-held Nerab, the Observatory and the pro-opposition TV station Orient reported. Three more people were killed in opposition-held Saraqeb, civil defense rescue workers said on a Twitter feed.

Syrian state media said five people had been killed in opposition rocket fire that hit regime-held Masyaf.

The foreign ministers of France, Germany, Britain, Canada, the US, Italy, and Japan on Saturday noted “with mounting concern the escalation in Syrian military activity in the de-escalation zone in Idlib over recent weeks,” according to a communique issued on Saturday after a Group of Seven meeting.

The Syrian regime has vowed to take back the northwestern region, comprising Idlib and adjacent areas of Hama and Aleppo provinces.

Last week, Damascus said its ally Russia had started to feel that its patience was running out over the northwest. However Moscow had told Damascus that Turkey was still determined to implement the agreement reached in September.

Turkey has deployed forces into Idlib under an agreement with Russia and Iran. Militant insurgents of the Tahrir Al-Sham group hold sway on the ground.

The UN says Idlib and the adjacent areas are sheltering some 3 million people, half of whom have been uprooted from other parts of Syria by the war.

The UN humanitarian office OCHA has said the escalating violence had already killed 90 civilians in the Idlib region in March, nearly half of them children. The escalation pushed more than 86,500 people to flee their homes in February and March, it added.

At least 370,000 people have died in Syria since the civil war erupted in 2011 with the brutal repression of anti-government protests.