The number of fatalities caused by an alleged chemical attack on Tuesday on the town of Khan Sheikhun in northern Syria has risen to 72, including 20 children, the Syrian Observatory for Human Rights says.
Earlier, Russia's defence ministry said the gas had leaked from a rebel chemical weapons depot after it was hit by Syrian government air strikes.
The US has blamed the administration of Syrian President Bashar al-Assad for the attack.
The US, France and Britain have presented a draft resolution to the UN Security Council criticising the suspected chemical attack and demanding an investigation into it.
The two-page resolution could be voted on by the Security Council as early as Wednesday.
