Syrian government troops and allied forces are advancing in an offensive to gain ground in northwestern Idlib province, the country's largest remaining rebel-held territory.

The push has forced thousands of civilians to flee towards the border with Turkey in freezing winter conditions, according to humanitarian groups.

The operation's main target appears to be the rebel-held airbase of Abu Zuhour on the southeastern edge of Idlib, a province largely dominated by Hay'et Tahrir al-Sham (HTS), a former al-Qaeda affiliated group.

On Sunday, the UK-based Syrian Observatory for Human Rights, a monitoring group, reported that forces loyal to President Bashar al-Assad had "gained control over the town of Sinjar", less than 20km from the airbase.

The pro-government forces also want to secure the Damascus-Aleppo road that cuts through Idlib, which was captured by rebels in 2015.

Supported by Iran-backed militias and Russian air power, Assad's troops have since late October recaptured rebel-held territory in Idlib and also seized dozens of villages in the northern parts of the nearby Hama province.

'Big need'

Idlib is supposed to be one of the so-called de-escalation zones set up in Syria last year with the backing of Russia, Iran and Turkey.

The plan was aimed at halting fighting and offering safety to civilians in those four areas: Idlib province, East Ghouta, northern Homs province and the country's south.

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However, the Syrian government and its allies have not abided by the deal and continue to target all areas included in the deal, apart from the south.

As fighting continues, human rights groups have expressed concern that a full-blown government offensive in Idlib could cause large-scale destruction and further displacement.

Abdusselam El Sherif, a spokesman for IHH, a Turkish humanitarian group, said the push has already had a significant effect on civilians.

Speaking to Al Jazeera from the Turkish province of Kilis near the Syrian border, El Sherif said that 23,775 families - more than 100,000 people - had fled southern Idlib and eastern Hama this week due to heavy bombardment.

"The majority of them have reached the Syrian-Turkish border," he said.

"They are distributed in many camps," where there is "a big need" for food items and clothing, added El Sherif.

According to the UN, Idlib is home to an estimated 2.6 million Syrians, including many internally displaced people who have already fled fighting elsewhere in the country.