Barrels suspected to contain chlorine gas have been dropped on a town in Syria, a doctor and rescuers have said.
It is not clear who was responsible.
The attack took place in Saraqeb in Idlib province, near where a Russian military transport helicopter was shot down by rebels hours earlier.
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.
Dr Abdel Aziz Bareeh, who works in Saraqeb, told the BBC that two barrels of chlorine gas were dropped on the town late on Monday.
"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.
The organisation of volunteer rescue workers said it suspected the barrels contained chlorine but was unable to confirm it.
Chlorine is a common industrial chemical, but its use in weapons is banned by the Chemical Weapons Convention.
In 2013, the BBC found strong evidence suggesting residents of Saraqeb were subject to a chemical attack by government helicopters, something denied by the Syrian authorities.
Separately, clashes are continuing near Aleppo, where rebels have launched an offensive to break a government siege.