That's a lie.
Its also not out of the blue, he's been continuously using chemical weapons in hard to overtake strongholds with underground bunkers that can't be bombed.
There are reports everywhere explaining why Assad used chemical weapons for the 8th or 9th time.
Syria nerve agent attack: why it made sense to Assad
March 19, 2013:
More than two dozen people, including several Syrian soldiers, die in a sarin gas attack in northern Syria. The government and rebels each blame the other for the incident.
Aug. 21, 2013:
Hundreds of men, women and children are gassed with the banned nerve agent sarin in an attack near Damascus. The U.S. blames the Syrian government.
April 2015
Syrian groups in Idlib province say barrel bombs containing chlorine were dropped.
August and September 2016
Hospital officials and activists in Aleppo say chlorine gas is used in attacks.
Feb. 28., 2017:
Russia and China veto a U.N. Security Council resolution authorizing sanctions against Syria for chemical weapons use.
April 4, 2017:
An attack on a rebel-held city in northern Syria kills at least 86 people, 27 of them children. Autopsies on three Syrians who died after being brought to Turkey for treatment suggest sarin was used in the attack.