Doctors Without Borders (MSF) said it had treated civilians apparently exposed to a chemical agent in Marea, without saying what type or providing overall casualty figures.
The Syrian American Medical Society (SAMS) said its own doctors in the northern town had identified the agent as mustard gas.
Mamoun al-Khatib, a journalist and activist from Marea, said more than 50 artillery shells crashed into the centre of the town, an opposition stronghold in the northern province of Aleppo, before midday last Friday.
Khatib accused the Islamic State (IS) group, which has attempted to seize Marea in recent months, of firing the shells which he said emitted a "hideous stench".
"We went to the local field hospital, and the doctors told us there were cases of suffocation, severe coughing, redness in the eyes and face, and skin irritations," said Khatib, who heads Shahba, an anti-regime news agency.
"In total, more than 25 people were affected, four of them serious cases that were transferred to Turkey. They were all civilians," he said.
SAMS, which supports clinics in war-torn Syria, said a field hospital it operates in Marea treated "more than 50 civilians exhibiting symptoms of chemical exposure".
"Roughly 30 civilians developed skin blisters, with doctors identifying the agent to be mustard gas," it said in a statement, adding blood, clothing and hair samples had been collected from patients for further assessment.
- 'Exposure to chemical agent' -
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MSF, a Paris-based medical humanitarian group, said it treated four patients, all from a single family in Marea, who were "exhibiting symptoms of exposure to chemical agents" in northern Syria on August 21.
The family arrived at MSF's facilities an hour after a mortar shell hit their home and yellow gas filled their living room.
"MSF has no laboratory evidence to confirm the cause of these symptoms," Pablo Marco, the group's programme manager in Syria, said in a statement.
"However, the patients' clinical symptoms, the way these symptoms changed over time, and the patients' testimony about the circumstances of the poisoning all point to exposure to a chemical agent."
Mustard gas is an asphyxiant that has been banned in war by the UN since 1993.
Syria's government had stockpiles of more than 19 tonnes of mustard gas, but it handed them over as part of a multinational effort to strip the country of 1,300 metric tonnes of chemical weapons.
Nearly two weeks ago, reports emerged IS jihadists in Iraq may have used mustard gas against Iraqi Kurdish fighters.
A US official said it was "plausible" the extremist group had used the deadly gas on Kurdish peshmerga fighters.
IS, which has seized territory across Syria and Iraq, is fighting Kurdish forces and Arab groups opposed to it in both countries.
In Syria, the jihadist group has fiercely attacked anti-government forces in Marea in an effort to cut off a supply route into the country from Turkey.