The third suspected chemical attack was reported to have been carried out in Syria on Tuesday over the province of Idlib, killing at least over 50 people, including children.
The US has accused the Assad government of carrying out this attack, however, the military has denied this.
"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 said in a statement.
Local aid workers and human rights activists have pointed to victims foaming at the mouth, with pin-point pupils and struggling to breathe as signs of the chemical gas sarin.
The White House described the attack as an "intolerable act", whilst the UN's Syria envoy said it was "horrific".