A Syrian rescue service operating in rebel-held territory says a helicopter dropped containers of toxic gas overnight on a town in Idlib province close to where a Russian military helicopter was shot down hours earlier.
33 people were affected, mostly women and children.
The Syria Civil Defense, which says it's a neutral band of search and rescue workers, says the gas attack happened near where a Russian military helicopter was shot down just hours earlier.
Russia says its helicopter was downed near Saraqeb and that all five people on board were killed.
Russian air power has been supporting President Bashar al Assad in Syria's five-year-long civil war.
Both the Syrian Government and opposition forces have denied using chemical weapons during the conflict.
Neither the Syrian government nor its Russian allies were immediately available for comment.