Rebel officials denied using chemical weapons and accused the Damascus government of trying to frame them.
There was a stench of gas in Aleppo city after projectiles were fired, said Rami Abdurrahman, the head of Britain-based Syrian Observatory for Human Rights.
The reported attack on Aleppo is the first such attack since Russian Federation and Turkey agreed on establishing a demilitarized zone in the rebel-controlled Idlib province, according to the Observatory.
The militant attack hit al-Khalidiya neighborhood in the government-held city on Saturday, foreign news agencies and state television reported, saying close to 100 people have been hospitalized.
"We cannot know the kinds of gases but we suspected chlorine and treated patients on this basis because of the symptoms", Zaher Batal, the head of the Aleppo Doctors Syndicate, told Reuters.
A health official in Aleppo said victims suffered breathing difficulties, eye inflammation and other symptoms suggesting the use of chlorine gas.
Russian Federation claimed that the chemical attack was performed through mortars.
But it is the first instance since the state media accused the rebels of firing shells with poisonous gas as the namesake capital city of Aleppo was fully liberated from the rebels in 2016 except the western and northern countryside. "This is purely a lie", he tweeted. Syrian state TV later said the attack has also hit two other areas in the city and said a total of 21 people had been injured.
"Assad is interested in undermining the cease-fire,"a spokesman for the organization said".
The UK-based Observatory said government shelling earlier on Saturday had killed two women and seven children in a village in Idlib.
Earlier this month, Moscow accused insurgents of trying to wreck the deal, while rebels accused the Syrian army and its allies of attacking the region.
The September 17 deal was meant to protect three million inhabitants in the Idlib region, more than half of which is held by an alliance led by jihadists of Al-Qaeda's former Syria affiliate.
Hayat Tahrir al-Sham alliance and the al-Qaeda-linked Hurras al-Deen group operate in the area, neither of whom have commented on the alleged attack.
A past UN-OPCW inquiry found the Syrian government used the nerve agent sarin in an April 2017 attack and has also used chlorine several times. She added that all parties, including the Assad regime and Russian Federation, should facilitate an investigation by the Organization for the Prohibition of Chemical Weapons into the suspected attack in Aleppo.
The latter attack, which came in response to what the U.S. and its allies had insisted was another chemical attack in Douma on the outskirts of Damascus, saw 105 cruise missiles being fired from sea and air at Syrian government targets.