Barrels that may have contained chlorine gas were dropped over a town in Idlib province in north-western Syria, a volunteer rescue worker told dpa on Tuesday.
"On Monday evening, Saraqeb city was targeted by two barrels dropped by a helicopter, each containing five canisters full of chlorine in addition to iron balls of various sizes," Rady Saad, a member of the non-governmental emergency group the White Helmets, told dpa.
"A total of 33 people were admitted to hospital, among them 10 children suffering from breathing difficulty," he said, adding that all admitted cases were documented by name.
There are still three cases in critical condition, among them a toddler, according to a statement by the White Helmets.
Saad said he could not verify whether the helicopter was the Syrian regime's or Russia's.
Chlorine gas can cause victims to have breathing problems and to foam blood from the mouth.
"The people there noticed the smell when the barrels were dropped over the area," Saad said.
Pictures of two toddlers receiving breathing treatment at a hospital were emailed to dpa by the volunteer group.
Meanwhile, the Syrian regime accused rebels of firing shells containing poisonous gas on neighborhoods controlled by the regime in the western sector of Aleppo.
"The government hospitals have received six people who have died from inhaling poisonous gas from the rebel shelling on al-Akabeh neighbourhood," Mohamed Hazouri a health official in the province of Aleppo, told dpa.
Adnan Abu Marya, a rebel opposition commander in eastern Aleppo, denied such accusations and said the rebels had not fired any shells on the Old Aleppo neighbourhood.
"Those are pure lies from the regime side and a cover-up for the regime use of chemical weapons on rebel areas," Abu Marya told dpa.
The Syrian government and rebels have been accused of using chemical agents banned by international community, but both sides have denied their use.
At least 30 civilians were killed in shellfire by Syrian rebel groups advancing in the south-western districts of Aleppo over the last 24 hours, the Syrian Observatory for Human Rights said Tuesday.
The heavy shelling, which continued until morning, came as regime forces overnight counter-attacked and won back some positions, the Britain-based observatory said.
State media said the Syrian air force carried out several strikes on areas controlled by "terrorists," a term used by the regime to describe the rebels.
But opposition figures remained upbeat, with Brita Hagi Hassan, the head of the opposition city council, telling dpa via a Facebook message: "If the rebels continue their push into regime areas in south-western Aleppo ... all of Aleppo will be liberated in one week."
Rebels led by the Fatah al-Sham Front, previously known as the al-Nusra Front before it broke with the al-Qaeda terrorist organization last week, launched a major offensive on Aleppo's south-western outskirts Sunday evening.
The rebel forces in the area are separated by only a few kilometres of regime territory from besieged eastern Aleppo.
Riad Kahwaji, founder and chief executive officer of the Institute for Near East and Gulf Military Analysis, told dpa that the opposition forces have surprised their opponents with their level of resilience.
"Despite the fierce offensive by the regime forces supported by Russian air cover and the Iranian-supplied Shiite militias, opposition forces managed to regain the momentum through a surprise counter-offensive from the west and south of Aleppo," Kahwaji said.
He added that the rebels seem to be on the verge of encircling regime forces in some pockets in and around the city.
"Air power can do so little in urban fighting, especially if the other side lacks smart weapons, and the Russian airpower is clearly short on effective precision weapons," Kahwaji said.
"I believe the battle of Aleppo is proving way much more difficult than the Iranian-Russian axis had expected; and it will be more costly to them if they keep trying to capture the city," Kahwaji warned.
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