The chemical attack in Syria's northwestern Idlib province killed more than 100 people, Turkish President Tayyip Erdogan said on Wednesday, without saying where the death toll number came from.

Erdogan, who was speaking at a rally in the northwestern province of Bursa, also called Syrian President Bashar al-Assad a "murderer". Ankara has long been one of Assad's most steadfast critics and has repeatedly called for his ouster.

Western countries, including the United States, have blamed Assad's armed forces for the chemical attack that choked scores of people to death in the town of Khan Seikhoun in a rebel-held area of northern Syria on Tuesday.

Russia, which backs Assad, denied on Wednesday the Syrian president was to blame for the attack, and said it would continue to support him.