About 30 people, mostly women and children, have been affected. Chlorine gas can cause victims to become short of breath and to foam blood from the mouth. The government and some rebel groups have been accused of, and denied, using chemical agents.
A Russian military helicopter was shot down near Saraqeb on Monday, killing five crew. It is not clear who brought it down. It was the biggest single loss of life suffered by Russian troops since they entered the conflict in Syria last year, on the side of President Bashar al-Assad.
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"We know it's chlorine because we were hit by it in the past and we are familiar with its odour and symptoms. "We have 28 confirmed cases, mostly women and children."
Separately, a spokesman for Syria Civil Defence told Reuters that 33 people had been affected in the attack.