He had earlier presented a report by weapons inspectors to the UN Security Council.
Ban Ki-moon called the attack a war crime and said the report made chilling reading. He added the evidence was “overwhelming and indisputable”
“It is the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja in 1988 and the worst use of weapons of mass destruction in the 21st century,” Ban said.
He confirmed that UN inspectors would return to Syria for other investigations but said it was now for others to decide whether to determine who was behind the attack