As many as 1,400 people were killed when several opposition-controlled areas in the suburbs around Damascus were struck by rockets containing the chemical agent sarin on Aug. 21, 2013. Both the opposition and the government traded accusations.

In the same year, a chemical attack hit the then government-controlled town of Khan al-Asal in the countryside of Aleppo, in which several Syrian soldiers and civilians were either killed or suffered from suffocation. The government accused the rebels, who, in turn, denied the accusation.

In October 2013, the Organization for the Prohibition of Chemical Weapons (OPCW) officials arrived in Syria to monitor the dismantlement of the Syrian chemical weapons arsenal, after Damascus officially joined the Prohibition of Chemical Weapons Convention.

The OPCW later said that the government has made its chemical weapon production facilities inoperable.

The dismantlement of the Syrian chemical weapons was due to a US-Russian understanding, the first sign of a consensus between both powers on the Syrian conflict.

Since then, reports of poisonous gas attacks kept emerging once in a while.