RED LINE REDUX: Rescuers say toxic gas dropped on Syrian town where Russian helicopter shot down.

A spokesman for Syria Civil Defence said 33 people, mostly women and children, were affected by the gas in Saraqeb, in rebel-held Idlib province.

The group, which describes itself as a neutral band of search and rescue volunteers, posted a video on YouTube apparently showing a number of men struggling to breathe and being given oxygen masks by people in civil defence uniforms.

Syria Civil Defence workers, who went to the scene of the attack, said they suspected the gas 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.

Another “strong verbal response” may follow from the White House.