The United Nations is investigating a suspected toxic gas attack on the besieged city of Aleppo, saying it would amount to a war crime if confirmed.
At least four people including a girl of eight and her ten-year-old brother were killed in the attack on the rebel-held Zubdiya area in the city.
A further 65 people - more than half of whom were children - were injured in the alleged chemical attack, doctors in Aleppo reported.
- Video report by ITV News Correspondent Paul Davies
Experts have said there is "strong evidence" that a banned substance - probably chlorine gas - was used by president Bashar al-Assad's forces.
Witnesses have described how children began "screaming and suffocating" amid a strong smell of gas.
Sama Raji, aged 8, and her ten-year-old brother Mahmoud were killed along with their mother in the strike.
Abdulkafi Alhamdo, who is inside the city, said his family had been among those caught up in the attack.
"I hardly could breathe," he told ITV News. "I was so afraid for my daughter."
He added: "It was so, so dangerous."
Staffan de Mistura, the UN's special envoy for Syria, said there "is a lot of evidence" that a chlorine attack did take place, adding that it would be classed as a war crime if verified.
Weapons expert Hamish de Bretton-Gordon agreed the attack was "almost certainly" chlorine from the descriptions given by doctors working in Aleppo.
Syria's government has denied using illegal chemical weapons against rebels.
Grim details of the suspected gas attack emerged as the UN called on president al-Assad and his Russian allies to respect a 48-hour ceasefire.
"The stakes could not be higher in the coming days - millions of Syrian civilians are now in a seeming freefall," Jan Egeland, the UN 's humanitarian adviser said.
Fighting has escalated in Aleppo in recent weeks, with the Russian-backed Assad regime engaged in an intense stand-off with rebels.
ITV News has documented the battle with many observers expressing fears of a humanitarian crisis with thousands trapped inside the city with scarce resources.
Russian forces announced they would suspend fighting for three hours a day to allow humanitarian aid in to the city.
However ITV News sources reported there had been shelling and bombardments today during the 'pause' period.
Meanwhile, the UN has said that breaks of at least 48 hours are needed to get in sufficient supplies.
Earlier, the last doctors in the rebel-held east of the city wrote to US President Obama to urge him to intervene to protect Syrian civilians and to stop the bombardment of hospitals.
Mr Obama was told to "act now to stop the bombs that continue to fall on the city and ensure they are never held under siege again”.