On Monday alone, the strikes killed 16 civilians including 11 in a vegetable market in the town of Saraqeb, the Britain-based Syrian Observatory for Human Rights said.

The 17 others were killed on Sunday in raids on various areas of the province, large parts of which are controlled by Hayat Tahrir al-Sham (HTS), which is dominated by Al-Qaeda's former Syria affiliate.

"Regime war planes have intensified their strikes over the past 24 hours after relative calm due to bad weather," Observatory chief Rami Abdul Rahman said.

Syrian troops had been advancing on Idlib as part of a fierce offensive launched in late December with Russian backing.

At the market in Saraqeb, an AFP correspondent saw pools of blood on the ground. Small trucks loaded with sacks of potatoes stood abandoned after their windows were blasted from their frames.

In front of a hospital in the town, a motorbike and a car were trapped below the rubble and twisted metal.

An alliance of jihadists and rebels overran the vast majority of Idlib province in 2015.

On January 21, Syria's army said it had captured the vital military airport of Abu Duhur on the edge of Idlib province, in a breakthrough for the government in the last Syrian province beyond its control.

With the airport's capture, the army said, troops would secure a key route leading from the neighbouring province of Aleppo south to the capital Damascus.

Syria's war has killed more than 340,000 people and displaced millions since it began in March 2011 with the brutal repression of anti-government protests.