A helicopter has reportedly dropped barrels suspected to contain chlorine gas on the northern Syrian town of Saraqeb.
A doctor and rescuers say some 30 people were affected by the attack, which occurred late on August 1.
It is not clear who was responsible.
Both sides in Syria's five-year-old civil war have been accused of using chemical agents.
Dr. Abdel Aziz Bareeh, who works in Saraqeb, said two barrels of chlorine gas were dropped on the town.
"We know it's chlorine because we were hit by it in the past and we are familiar with its odor and symptoms," Bareeh told the BBC.
"Medium-sized barrels fell containing toxic gasses," Reuters news agency quoted a spokesman for the Syria Civil Defense as saying. The spokesman added that the group was unable to determine the type of the gas.
On August 1, a Russian military helicopter was shot down near Saraqeb.
Based on reporting by Reuters and BBC