The Idlib region -- home to around three million people including many displaced by Syria's eight-year civil war -- is largely under the control of a group dominated by a former Al-Qaeda affiliate.

Air raids by President Bashar al-Assad's regime killed 13 civilians in a market in the Idlib town of Maaret al-Numan, said the Britain-based Syrian Observatory for Human Rights.

An AFP correspondent saw rescue workers carry a wounded man away on a stretcher while a body lay motionless amid spilt oranges and bags of onions.

Maher Mohammed, 35, a vendor in the market, said it was the most frightening bombardment he had witnessed in years.

"We ran inside the shops and threw ourselves on the ground," the father of five told AFP.

"They bombarded half the market. Our neighbours were killed," as were two women in a car who had come to do some shopping, he said.

The Observatory said two others died in Russian and regime bombardment in other parts of the province.

Regime forces and armed groups were locked in heavy clashes on the southeastern edge of the region, with almost 100 fighters killed in two days, the Britain-based monitor said.

The battles on the edge of Idlib since Saturday are the most deadly since a Russia-brokered ceasefire went into effect in late August, it said.

- Fighting rages -

"Fighting raged at dawn Monday on several axes in the southeastern Idlib countryside," the monitoring group said.

Fifty-four regime fighters had been killed, while 47 of their opponents including 33 jihadists had also lost their lives since Saturday, the Observatory said.

The Syrian government does not usually divulge casualty figures.

The Idlib region is one of the last holdouts of opposition groups to the forces backing Assad.

The president has long maintained that his government will eventually reimpose its control over the northwestern region on the border with Turkey.

The late August ceasefire came months into a devastating Russia-backed regime offensive that killed around 1,000 civilians and ousted hundreds of thousands from their homes.

But sporadic clashes and deadly Russian and regime bombardment on the jihadist-held bastion have continued, with 179 civilians including 45 children killed during that time.

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