At least 15 people were killed, 45 in last five days, in crossfire between the Syrian military and the rebel forces in northwestern Syria on Sunday.
Last week, Damascus said its ally Russia’s patience is running out over the northwest.
However, Moscow had told Damascus Turkey was still determined to implement agreement reached in September.
Moscow struck a deal with Ankara that included the creation of a “demilitarized zone”.
The northwest territory is the last major foothold of fighter groups opposed to President Bashar Al Assad.
The agreement has come under strain in recent weeks.
In the last five days alone 45 people were killed, according to reports.
The foreign ministers of European Union and the United States said, escalation in Syrian military activity in the de-escalation zone in Idlib over recent weeks will further cause a humanitarian catastrophe.
The Syrian Observatory for Human Rights said most of the killings were due to the Syrian military forces shelling of rebel-held areas.
Last year Damascus mounted an offensive into the northwest.
The Syrian military backed by Iran and Russia forced to retreat the rebel groups in many areas.
The Syrian government has vowed to take back the northwestern region, comprising Idlib and adjacent areas of Hama and Aleppo provinces.
Turkey has deployed forces into Idlib under an agreement with Russia and Iran.
Some 3 million people are in Idlib and the adjacent areas, half of whom have been uprooted from other parts of Syria.
Turkish forces control north of Aleppo territory where Turkey-backed Free Syrian Army rebels are based.