"What we have understood, it was a chemical attack and it came from the air," de Mistura told reporters in Brussels, adding that there should be a "clear identification of responsibilities and accountability.
De Mistura, fresh from the latest round of UN-sponsored peace talks in Geneva between the rebels and Damascus, said that every time there was a sign of progress, someone -- unnamed -- always tried to sabotage it.
"But we are not going to give up. On the contrary, we make use of all these horror moments to show they cannot prevail," de Mistura added.
Separately, the UN Commission of Inquiry for Syria said it had begun investigating the attack on a rebel-held town in northwestern Idlib Province.
"Reports suggesting that this was a chemical weapons attack are extremely concerning," it said in a statement.