An international medical organisation has said its doctors treated four patients last week in the Syrian city of Aleppo who were exhibiting symptoms of exposure to chemical agents.
Doctors Without Borders (MSF) said on Tuesday that the patients were all from one family - two parents, a three-year-old girl and a five-day-old baby girl.
In a statement on Tuesday, MSF said the four family members had arrived at one of its hospital one hour after an attack occurred on August 21.
The organisation said the patients were suffering from respiratory difficulties, inflamed skin, red eyes and conjunctivitis.
"Within three hours they developed blisters and their respiratory difficulties worsened," it said.
"MSF staff treated their symptoms and gave them oxygen before transferring them to another facility for specialised treatment."
The family came from the town of Marea, in Azaz district, north of Aleppo, which had been under intense bombardment by mortars and artillery for a week.
Marea is on the front-line of fighting between the Islamic State of Iraq and the Levant (ISIL) and the Islamic Front, an offshoot of the Free Syrian Army comprised of almost a dozen factions, including Ahrar al-Sham and al-Tawahid brigade.
The town is located along the highway linking Aleppo to the Turkish border, which makes it significant for ISIL as a route for transporting supplies and to bring in fighters.
The US previously accused ISIL of using mustard gas in July against Kurdish forces in northern Syria, in addition to two attacks in August against Iraqi Kurdish forces.
The Syrian regime of Bashar al-Assad is also accused of using chemical weapons against its rivals.
Mustard gas, also known as sulphur mustard, is a chemical compound which was first used as a chemical weapon during World War I.
The patients told MSF that a mortar shell hit their home at about 7:30pm on August 21 and a yellow gas filled their living room afterwards.
"MSF has no laboratory evidence to confirm the cause of these symptoms," said Pablo Marco, MSF's programme manager in Syria.
"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."
The development comes on top of a rapidly deteriorating humanitarian situation in Aleppo governorate, MSF said, adding that at least 11 medical facilities have been deliberately targeted with barrel bombs over recent months.
Warplanes and helicopters belonging to Assad's forces have frequently been accused of dropping barrel bombs on residential areas outside their control. Assad has denied his forces use barrel bombs.
MSF said it operates six medical facilities inside Syria and directly supports more than 100 clinics, health posts and field hospitals in the country.
Source: Al Jazeera