Syrian state media says toxic gas was contained in shells that rebels have been firing at government-held areas in the divided city.
Russian news agency Interfax says seven people died and more than 20 were hospitalised after Tuesday's attack.
There has been no independent confirmation of the claim.
It comes after reports from a rebel-held town in Idlib province that chlorine gas was dropped on it late on Monday after a Russian military helicopter had earlier been shot down nearby.
A battle has continued since the weekend in Aleppo, as rebels attempt to break a government siege of a rebel-controlled area.
It is expected to be decisive for the future of a city that was once a commercial heartland but has been destroyed by five years of war.
About a quarter of a million civilians are living under siege since government forces cut them off last month.
The latest reports from the area are that the government, backed by Russian jets, has regained some ground. It is not clear exactly which territory has returned to government control but a reporter for a Beirut-based TV station said the army had regained control of two out of three villages that it lost near Aleppo earlier this week.
Russian and Syrian forces say they have been operating seven so-called humanitarian corridors, allowing hundreds of people to leave the besieged area peacefully.
On Wednesday, Russian state television ran pictures of civilians and fighters leaving. Smoke was seen billowing over the city in the footage, and gunshots were heard.
The UN's Children's Fund (Unicef) said it is "extremely" concerned for the safety of children caught up in violence in the city.
Saad Houry, of the fund, said children make up a third of the population trapped in the siege, and called for unhindered humanitarian access and for children to be protected.
The New York-based Physicians for Human Rights group said the last week of July was the worst week for attacks on medical facilities in the region since the Syrian war began in 2011. In a statement, the group said it has verified six deadly air strikes against hospitals in Aleppo governorate, all perpetrated by Syrian government forces.
Widney Brown of Physicians for Human Rights said: "Destroying hospitals is tantamount to signing thousands of death warrants for people now stranded in eastern Aleppo."

The book, which upholds the right to ridicule religion, was finished two days before Charb was killed by Islamic militants in January, publishers say.
It argues that the fight against racism is being replaced by a misguided struggle against "Islamophobia".
Charb and 11 others were killed during a Charlie Hebdo editorial meeting.
The attack on the Paris offices of the newspaper was carried out by two brothers, Said and Cherif Kouachi, who were later shot dead by police.
Charb had received numerous death threats following Charlie Hebdo's publication of cartoons featuring the Prophet Muhammad in 2006. The magazine's offices were firebombed in 2012.
Charb's book - which goes on sale on Thursday - is entitled An Open Letter to the Fraudsters of Islamophobia who Play into Racists' Hands.
It is both a defence of Charlie Hebdo's editorial stance and an attack on the paper's detractors.
"The suggestion that you can laugh at everything, except certain aspects of Islam, because Muslims are much more prickly that the rest of the population - what is that, if not discrimination?"
He condemns this position as "white, left-wing bourgeois intellectual paternalism".
Charlie Hebdo, which was launched in 1969, poked fun at conservatives and all religions. It had a small circulation and folded in 1981, but was resurrected in 1992.
The Russian military says it has informed the US that it believes rebels in the Syrian city of Aleppo have deployed "toxic substances".
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A book written by the late editor of French satirical weekly Charlie Hebdo, Stephane Charbonnier - known as Charb - is set to be published posthumously.