At least 16 children have been slaughtered as they left school after an air strike on a rebel-held province in northwestern Syria today.

The children, who were among 20 people killed in the attack on Kfar Batkeeh village, were aged between seven and ten. 

They took refuge in a cave when jets began flying overhead, local activist Raghda Ghanoum said, and stayed there as the bombs fell. 

The violence in both government-held and opposition-held areas came as Syrians celebrated Mother's Day, turning the occasion that ushers the spring season into a blood-spattered day for families on both sides of the conflict.

The UK-based Syrian Observatory for Human Rights estimated at least 20 people were killed in the air strike on the village in Idlib province.

Idlib is the largest, most populous area still held by rebels seeking to oust Syrian President Bashar al-Assad, whose forces are backed in the war by Russian air power. 

The Observatory put the death toll of an earlier shelling on a market in Damascus at 43, including 11 pro-government fighters.

Hospital Director Mohammed Haitham al-Husseini told Al-Ikhbariya TV that 35 others were wounded in the mortar attack, with six in intensive care. He said most of the casualties were women and children. 

The assault on eastern Ghouta has displaced 45,000 people, the United Nations said Tuesday. Before the latest offensive, it was estimated that 400,000 people were trapped in the besieged region. 

The rebels first seized the area in 2012.

The Syrian Civil Defense said that since the latest offensive started a month ago, it documented 1,252 civilians killed in more than 2,990 airstrikes and hundreds of other shellings.

Government forces have made major gains in recent days, leaving just a small fraction of eastern Ghouta under rebel control. President Bashar Assad paid a rare visit to troops on the front lines over the weekend. 

Syrian and Russian forces have opened a third corridor in eastern Ghouta to allow civilians to leave the town of Harasta, which is home to an estimated 20,000 people. 

Russia's Maj. Gen. Vladimir Zolotukhin said 300 civilians and 15 militants have exited through the corridor in the past 24 hours.