A Syrian military helicopter has been shot down over the last major rebel bastion in the northwest of the country, the second such incident in a week of high tensions with neighbouring Turkey.

The attack on Friday in a region where Turkish troops and Russian-backed government forces have engaged in multiple clashes came as Washington urged Ankara to look to its western allies in light of Moscow’s actions.

Syrian and Russian forces have conducted a deadly offensive against the shrinking pocket in the country’s northwest since December, claiming the lives of eight civilians on Friday.

“At approximately 1:40pm (local time), one of our military helicopters was hit by a hostile missile in the western countryside of Aleppo,” Syrian state news agency SANA said.

“This led the helicopter to crash, killing all crew on board.”

It said the aircraft was downed near the town of Urum al-Kubra, where Turkey-backed rebels operate, but did not say who fired the missile.

The Turkey-backed National Liberation Front rebel group claimed responsibility in a statement posted on the Telegram app.

The Syrian Observatory for Human Rights said the helicopter’s two pilots were killed.

Three days earlier, another Syrian military helicopter was downed over Idlib province, killing at least three crew members.

Turkish media blamed that attack on rebels, but the Observatory said Ankara’s troops had fired rockets at the aircraft over the village of Qaminas, southeast of Idlib city.

Turkey did not claim responsibility.

Since December, Syrian government forces have undertaken a blistering assault on the last major rebel pocket in the northwestern Idlib region and parts of neighbouring Aleppo and Latakia provinces.

Syrians meet Turkish troops outside their base near the village of Binish, in Idlib province. Turkey, which backs Syria’s opposition, has been deploying equipment and troops in the region, to halt the Syrian military’s advances. Photograph: Ghaith Alsayed/AP

The Observatory said eight civilians, including three children, were killed in the bombardment on Friday, adding that five died in Russian raids near the almost deserted city of Atareb.

Government forces are within 5km of the city, the monitor said.

The assault has sparked the largest wave of displacement in the nine-year conflict, with 800,000 people fleeing since December, according to the United Nations. Among them, some 82,000 people are sleeping rough in sub-zero temperatures.

European members of the UN Security Council called Friday for an immediate end to the Idlib offensive to save lives.

“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.

“There can be no sustainable military solution to the Syrian conflict,” they said in a joint statement.

The unprecedented exodus of civilians has triggered alarm in Turkey, which fears an influx across its border.

Turkey first sent troops to Syria in 2016 and has sent reinforcements to the northwest in recent weeks to contain the assault.

That has led to a series of confrontations, including deadly clashes this week that saw government shelling kill five Turkish troops.

Turkish President Recep Tayyip Erdogan has threatened the Syrian government in recent days, saying the offensive violates a 2018 deal with Russia meant to prevent a broad military operation.