A Syrian rescue service operating in rebel-held territory said on Tuesday a helicopter dropped containers of toxic gas overnight on a town close to where a Russian military helicopter was shot down hours earlier.
A spokesman for Syria Civil Defence told Reuters that 33 people, mostly women and children, were affected by the gas in Saraqeb.
The group, which describes itself as a neutral band of search and rescue volunteers, posted a video on YouTube purportedly showing a number of men struggling to breathe and being given oxygen masks by people in civil defense uniforms.
Syria Civil Defence workers, who went to the scene of the attack, said they suspected it was chlorine but could not verify that.
"Medium-sized barrels fell containing toxic gasses. The Syrian Civil Defence was not able to determine the type of the gas," said the spokesman.
The Syrian government and its Russian allies were not immediately available for comment.
Both government and opposition forces have denied using chemical weapons during the five-year-old civil war. Western powers say the government has been responsible for chlorine and other chemical attacks. The Syrian government and Russia have accused rebel forces of using poison gas.