UN Syria envoy Staffan de Mistura said Tuesday that a "horrific" attack on a rebel-held town in the country that killed at least 58 people was believed to be chemical and launched from the air.

"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.

"Every time we have a moment in which the international community is capable of being together, there is someone, somehow that tries to undermine that feeling of hope by producing a feeling of horror and outrage," he said on the sidelines of a Syrian aid conference in Brussels.

"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.

The Organisation for the Prohibition of Chemical Weapons (OPCW), based in The Hague, meanwhile also said it was "gathering and analysing information from all available sources" about the attack.

source: AFP