Turkey's military has sent additional arms and troops to Idlib, on its southern border, to confront a push by Russia-backed Syrian government forces to retake the country's last major rebel stronghold after almost nine years of war.
Buoyed by their recapture of the main highway linking Syria's major cities this week, government forces seized a key base west of second city Aleppo on Friday that they had lost to rebels in 2012, the Observatory said.
"One of our military helicopters was hit by an enemy missile in the western countryside of Aleppo.where armed terrorist organisations supported by Turkey are deployed, and this led to the fall of the helicopter and death of its crew", Syrian state news agency SANA cited a military source as saying.
The deadly clashes, the latest of which saw government shelling kill five Turkish troops this week, are the most serious since Ankara first sent forces to Syria in 2016. SANA said the aircraft was downed near the town of Urum al-Kubra, where Turkey-backed rebels operate, but did not say who was behind the attack.
The Syrian Observatory for Human Rights blamed the attack on rebel-backer Turkey but there was no immediate claim of responsibility.
Three days earlier, another Syrian military helicopter was downed over Idlib province, killing at least three crew members.
Ankara acknowledged the incident in a statement but did not claim responsibility.
Syrians meet Turkish troops outside their base near the village of Binish, in Idlib province.
The Observatory said eight civilians, including three children, were killed in the bombardment on Friday, adding that five died in Russian raids near the nearly deserted city of Atareb.
Among the displaced, some 82,000 people are sleeping rough in sub-zero temperatures.
Government forces are within 5km of the city, the monitor said.
"We demand that the parties, especially the Syrian regime and its allies, immediately end their military offensive, establish a genuine and lasting ceasefire", said permanent member France and non-permanent members Belgium, Estonia and Germany.
The unprecedented exodus has sparked alarm in neighbouring Turkey, which fears an influx across its border.
It has sent reinforcements to the northwest in recent weeks to contain the latest push by Damascus, leading to a series of tit-for-tat exchanges between their forces.
Turkish President Recep Tayyip Erdogan has threatened the Syrian government in recent days, saying the offensive violates a 2018 deal with Russian Federation meant to prevent a broad military operation.