The devastating gas attack on Syrian civilians that took place in August 2013 killed more than 1,400 people — including 426 children, it was revealed Sunday.

The extent of the assault in a Damascus suburb — which the UN secretary-general called “a crime against humanity” — was disclosed in a new US intelligence estimate, according to a “60 Minutes” report.

“Nobody knew what was going on,” witness Kassem Eid told the show. “People were just praying for God to have mercy on them.”

Eid captured the atrocity on video, which shows victims — including small children — flailing desperately as gas takes over their respiratory systems.

“I’ve seen things you won’t even dream about in your worst nightmares,” Eid said.

Millions have fled the country during the ongoing civil war.

The atrocity has been blamed on the regime of Bashar al-Assad, because the targeted town had rebelled against him.

Assad has denied that he ordered the strike.