Some 24 people reportedly suffered breathing difficulties in Saraqeb, a town 50 kilometres (30 miles) south of Aleppo, after a barrel bomb attack on Tuesday, according to the Syrian Observatory for Human Rights.
Residents said chlorine gas had been used in the attack, but the Britain-based Observatory could not confirm this.
"These reports are of great concern," Ahmet Uzumcu, the head of the Organisation for the Prohibition of Chemical Weapons (OPCW), said in a statement.
The UN-backed group based in The Hague "continues to examine any credible reports" of chemical weapons use, he added.
The incident took place close to where Russia said on Monday one of its military helicopters was shot down, killing the five people on board.
Uzumcu added that under international conventions the use of chemical weapons "by anyone under any circumstances" is seen "as reprehensible and wholly contrary to the legal norms established by the international community".
Syria's conflict has killed more than 280,000 people and drawn in world powers since it erupted in March 2011, as protests calling for the ouster of President Bashar al-Assad's regime escalated into all-out civil war.
In January, the OPCW announced all Syria's declared chemical arms stockpile had been completely destroyed.
But concerns remain that undeclared amounts of sarin gas and other chemical weapons have still been used in the conflict.
A joint investigation set up by the UN probing nine chemical weapons attacks in Syria in 2014 and 2015 is due to report its findings this month.
A still image taken on August 2, 2016 from a video said to be taken on August 1, 2016 posted on social media shows two men standing over a man on a bed, making him sit up as he breathes through a mask in what is said to be in Saraqeb, Idlib province, Syria.
The world's chemical weapons watchdog Wednesday voiced concern over reports of a chlorine gas attack near the battleground Syrian city of Aleppo.
