Russia has clashed with other world powers at the United Nations Security Council in New York on the chemical weapons deaths in northern Syria.
BBC report that Moscow’s suggestions that civilians were poisoned by rebel weapons on the ground have been widely rejected.
The UK’s foreign secretary, a rebel commander and a weapons expert all said the evidence pointed to an attack by Syrian government forces.
The issue is also overshadowing an aid conference on Syria in Brussels.
Seventy donor nations are discussing aid efforts in the war-ravaged country. The UN’s Emergency Relief Coordinator, Stephen O’Brien, said details of new pledges would be released later on.
According to UK-based monitoring group the Syrian Observatory for Human Rights, 20 children and 52 adults were killed in the chemical incident in Khan Sheikhoun, Idlib province, on Tuesday.
Opening the Security Council meeting, UN High Representative for Disarmament Affairs Kim Won-soo said that, if confirmed, this would be the biggest chemical weapons attack in Syria since Ghouta in August 2013, when hundreds of people were killed in rebel-held suburbs of Damascus.
Some of the victims were treated across the border in Turkey. One woman in hospital said: “We were affected by the gas. We couldn’t stand up. I felt dizzy and sick. I suffer from shortness of breath. I couldn’t breathe.”
The World Health Organization said some of the victims had symptoms consistent with exposure to nerve agents.
A team from medical charity MSF treating victims in Idlib found patients’ symptoms were “consistent with exposure to a neurotoxic agent such as sarin gas”, the charity said in a statement.
Sonia Khush, Syria director of the charity Save the Children, said victims had travelled far and wide to get treatment, making it difficult to estimate how many had been affected.
The government of Syrian President Bashar al-Assad, which is backed by Russia, denies its forces launched a chemical weapons attack.
Russia has acknowledged that Syrian planes did attack Khan Sheikhoun but it says the aircraft struck a depot producing chemical weapons, for use by militants in Iraq.
“Yesterday [Tuesday], from 11:30am to 12:30pm local time, Syrian aviation made a strike on a large terrorist ammunition depot and a concentration of military hardware in the eastern outskirts of Khan Sheikhoun town,” Russian defence ministry spokesman Igor Konoshenkov said.
“On the territory of the depot there were workshops which produced chemical warfare munitions.”