Syria's army and its allies pressed further into the country's largest remaining rebel stronghold on Sunday, capturing a town and several villages as they approached a military airport, a pro-Damascus media unit and a war monitor reported.

Forces loyal to Bashar Al Assad have stepped up the offensive in the southern province of Idlib in recent days, advancing towards the Abu Al Duhur airport, which rebels captured from the president's troops in September 2015.

The Syrian army and its allies "have gained control over the town of Sinjar," 14 kms from Abu Al Duhur, and three villages to the west, the media unit run by Assad ally Hizbollah reported.

Supported by Iran-backed militias and Russian air power, Assad's forces have since taken back rebel-held territory in Idlib and the northeastern province of Hama since late October.

The Syrian Observatory for Human Rights said on Sunday that the army had taken more than 95 villages in Hama and Idlib since October 22, including around 60 in Idlib alone during the past 14 days.

"Battles have shifted now to the northwest of Sinjar after the Syrian army and its allies have controlled the town," the Britain-based monitor reported.

The main rebel force in Idlib is Tahrir Al Sham, spearheaded by the former Al Qaeda affiliate in Syria previously called Nusra Front.

The Syrian army lost Idlib, which borders Turkey, to insurgents when the provincial capital fell in 2015. It became the only province fully under opposition control.

The largest population gathering is in Eastern Idlib, the monitor said, expressing fears for further intensive civilian displacement in the coming few days due to the ongoing fighting.

Tens of thousands of rebels and civilians took refuge in Idlib after abandoning their homes in other parts of western Syria that the government and its foreign allies have retaken.

The fighting and air strikes have forced more than 60,000 people to leave their homes since November 1, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA).

The United Nations said the civilians newly displaced by the fighting in Idlib were in a "dire" situation.

On Saturday regime and Russian air strikes on a rebel-held enclave near the Syrian capital killed at least 17 civilians.

Eastern Ghouta, one of the last remaining opposition strongholds in the country, is the target of near-daily air raids.

The Syrian Observatory for Human Rights said Saturday's deadliest strikes had hit the Hammuriyeh district, killing 12 civilians, including four children.