Seven civilians died in air strikes in the south of the bastion, the Syrian Observatory for Human Rights said, a day after regime air strikes killed 12 others, including six children in a single town.

Eight years into Syria's war, the Idlib region of some three million people is one of the last holdouts of opposition to President Bashar al-Assad's regime.

His forces have been chipping away at the southern edge of the stronghold run by Syria's former Al-Qaeda affiliate for three weeks, after months of deadly bombardment.

Last week, the regime took control of the town of Khan Sheikhun on a key highway that runs through Idlib province, linking Damascus to second city Aleppo.

Overnight, pro-government fighters "managed to advance in the southern Idlib countryside", according to the head of the Britain-based Observatory, Rami Abdel Rahman.

They took control of the towns of Al-Tamaanah and Al-Khuwayn, as well as nearby villages, east of Khan Sheikhun, he said.

"The forces are trying to further extend their control in the area of Khan Sheikhun before they advance north in the direction of the town of Maaret al-Noman," he added.

Maaret al-Noman is the next stop northwards on the Damascus-Aleppo highway.

- Children killed -

Regime air strikes killed 12 civilians -- half of them children -- in Maaret al-Noman on Wednesday.

AFP photographers at the scene saw rescue teams working to pull people from the rubble after nightfall with the help of bright spotlights.

A rescue worker pulled a very young girl from the debris, her lifeless body covered in white dust.

Her bloodied head and little arm hung limp as he carried her in his arms to an ambulance.

Inside a morgue, a young father sobbed over the bodies of his three dead children and their cousin, none of whom appeared older than five.

Also on Wednesday, another four civilians, including one child, were killed in other parts of the bastion.

Increased bombardment by the Syrian military and Russia since late April has killed more than 950 civilians in Idlib.

In the same period, the violence has displaced more than 400,000 people including many already uprooted from other areas, the United Nations says.

The bombardment and ground offensive come despite a deal between Russia and rebel backer Turkey in September last year to protect the area with a planned buffer zone.

That security cordon was never fully implemented, but Turkish troops deployed to 12 monitoring points around the planned buffer.

On Wednesday, the Observatory said air raids hit near one of the monitoring points in Sheir Maghar.

Turkey's state-run Anadolu news agency said the post itself was not hit.

Syria's war has killed more than 370,000 people and displaced millions more since it started in 2011 with the brutal repression of anti-government protests.