The Russian military says it has informed the US that it believes rebels in the Syrian city of Aleppo have deployed “toxic substances”.
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.”