Syrian forces and allies are advancing on the largest remaining rebel-held area in the north of the country, forcing thousands of civilians to flee towards the border with Turkey in freezing temperatures.
The offensive on Idlib – a large province in north-west Syria packed with civilians and dominated by al Qaida-linked militants – was expected after the defeat of Islamic State late last year.
Government forces recaptured the town of Sinjar, about 12 miles south of Abu Zuhour airbase, on Sunday.
“Conditions on the ground are wretched for the rebels,” said an opposition activist.
He said rebels are stuck in a two-front battle with government forces and remaining pockets of IS militants, and Russian air strikes have taken a heavy toll.
Opposition activists say the main target for now appears to be the rebel-held airbase of Abu Zuhour, on the south-eastern edge of the province, and securing the Damascus-Aleppo road that cuts through Idlib.
Another opposition activist, Mohammed al-Ali, said the Russians and the Syrian government are carpet-bombing villages before pushing into them.
“The Russian air strikes, weak fortifications and Islamic State attacks in Hama” have all helped government forces, he said.
It is reported to be highly unlikely that government forces would march toward the provincial capital, also named Idlib, as it would set up a costly battle with highly experienced and well-armed al Qaida-linked insurgents.
The province is dominated by the Levant Liberation Committee, which claims to have severed ties with al Qaida but is widely believed to still be affiliated with it.
Last week, government forces advanced to within around eight miles of Khan Sheikhoun, where a sarin nerve gas attack killed more than 90 people last year, prompting the US to launch a missile attack on President Bashar Assad’s troops.
Experts from the UN and other monitoring groups blamed the chemical attack on the government, which denied responsibility.
The Britain-based Syrian Observatory for Human Rights, which monitors the fighting through a network of activists, says some 43 civilians, 57 militants and 46 pro-government forces have been killed since the offensive began on December 25.
The Idlib offensive carries significant risks.
The province bordering Turkey is home to more than 2.6 million Syrians, according to the UN, including more than 1.1 million who fled fighting elsewhere in the country. A full-blown government offensive could cause large-scale destruction and massive displacement.
Turkey, a supporter of the rebels, has deployed military observers in the province as part of a de-escalation deal with Iran and Russia, but that has not stopped the fighting on the ground or Russian air strikes against the insurgents.