The UN says there's evidence chlorine gas has been dropped on civilians, as hundreds of thousands of Syrians remain trapped in the besieged city of Aleppo.
The use of chemical weapons would add yet another strike to the long list of war crimes committed against the Syrian people by their own government.
Hospitals in Aleppo released several pictures along with a strong claim from medical staff who say a banned chemical agent, probably chlorine gas, was used against the civilian population last night.
One man said he heard bombs drop then there was a strong smell of gas. Children began screaming, then suffocating.
Hospital CCTV captures the moment the first victims began to arrive, with more than 60 people experiencing breathing problems and more than half of them children.
All the victims were given oxygen, including small babies. Fifteen infants less than a year old were treated - all survived.
Rescue teams first at the scene of the bombing believe they have recovered evidence of a war crime.
They say mangled metalwork uncovered is what remains of barrel bombs that contained the gas and were dropped from government helicopters onto the rebel-held district of A Zubdiyah.
Abdul-Khafi al Hamdo who regularly sends eye witness accounts from inside Aleppo for ITV News, went to the scene by daylight.
Amid the destruction, he discovered the fate of pets unable to escape the gas.
Aleppo hospitals have reported four deaths including brother and sister Sama and Mamoud Raji and their mother.
One boy is among those who are seriously ill, showing apparent symptoms of chemical poisoning - but they can't be evacuated.
Heavy fighting continues around Aleppo and a three-hour ceasefire Russian forces promised to introduce on Thursday was never going to be long enough to organise a safe route in and out of this city that's become a battlefield.
The UN is pressing Russia for a 48 hour halt to hostilities while promising to investigate what it describes as considerable evidence that Mr Assad's forces last night committed a war crime.