Several people were killed in suspected chlorine attack on besieged city earlier this week
The UN's special envoy for Syria has said that a chemical attack on Aleppo, if confirmed, would amount to a war crime.
Staffan de Mistura was responding to reports that several people were killed after forces loyal to President Bashar al-Assad dropped chlorine gas on rebel-held parts of the city.
"It's really not for me to assess who did it and whether it actually took place, although there is a lot of evidence that it actually did take place," Mr de Mistrua said.
"We have a special UN and other organisations addressing that. But if it did take place, it is a war crime."
Footage of the attack published by the Aleppo Media Centre, an opposition news portal, showed a child and adults wearing breathing apparatus.
Locals reported smelling gas and seeing barrel bombs before people began to suffer breathing and eye problems.
The Syrian-American Medical Society estimates that almost 1,500 people have been killed in chemical weapons attacks since civil war broke out five years ago.
Both sides deny using chemical weapons, but UN investigators say sarin gas was used three years ago in Eastern Ghouta, near Damascus, killing an estimated 1,429 people.
Mustard gas was also used in August 2015, according to the UN watchdog Organisation for the Prohibition of Chemical Weapons.
'Workable humanitarian pause'
During yesterday's briefing, UN officials also criticised Russia’s proposed ceasefire of three hours a day, calling for a break of up to 48 hours instead.
UN special adviser Jan Egeland said the proposed pause was "not enough" to facilitate the delivery of aid to the estimated two million civilians trapped in the city.
"We need 48 hours; that’s how we see it now and we want to sit down with the Russian side," he said.
"We need a pause in the fighting that has to be guaranteed by the Russians, the US, the government (of Syria) and the armed opposition groups.
"We need 48 hours, because the people are so many, the convoys have to be picked, the roads are so destroyed, there are so many dangers, the logistics are so enormous that we need time each week and we need 48 hours."
Speaking after meeting Russian officials, Mr Egeland told reporters the UN was working to agree a "workable humanitarian pause" in fighting.
"The Russian delegation confirmed their willingness to sit down with us (on Thursday and Friday) to try to agree on a workable humanitarian pause for us to go the Aleppo road way to help the poor people of east as well as in the west," he said.