Foaming at the mouth, dilated pupils and difficulty in breathing are among the symptoms experienced by victims of an attack on civilians and medical staff in Syria on April 13 - which is now thought to have been caused by chemical weapons.
British, French and Israeli intelligence have backed claims by opposition fighters battling Syrian President Bashar al-Assad's forces that chemical weapons had been used in Allepo.
But while Syrian state television has said that the opposition fighters are to blame, the US now says that its intelligence assessments show, with varying degrees of confidence, that Assad's regime has used sarin gas.
Al Jazeera's Sue Turton spoke to Dr Niazi Habash who treated some April 13 attack victims. He said the symptoms indicated use of chemicals.