Up to 100 men, women and children have died and dozens more battling injuries after the deadliest chemical weapons attack in Syria since 2013.
Syrian forces are suspected to be behind the attack on the rebel-held town of Khan Sheikhoun in the Idleb province, but the Syrian military have denied responsibility, saying they would never use chemical weapons.
"We deny completely the use of any chemical or toxic material in Khan Sheikhoun town today and the army has not used nor will use in any place or time neither in past or in future," the Syrian army command said in a statement.
The World Health Organisation (WHO) confirmed in a statement released on Tuesday the symptoms of those affected by the attack appear to show signs the chemicals used included deadly nerve agents banned by international law.
"The likelihood of exposure to a chemical attack is amplified by an apparent lack of external injuries reported in cases showing a rapid onset of similar symptoms, including acute respiratory distress as the main cause of death. Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents," the statement read.
Dr Peter Salama, Executive Director of the WHO Health Emergencies Programme, said: "The images and reports coming from Idleb today leave me shocked, saddened and outraged. These types of weapons are banned by international law because they represent an intolerable barbarism."