United Nations war crimes investigators on Wednesday said they had evidence that Syrian government forces were behind the chemical attack that killed dozens of people in Khan Sheikhun in April.

In the first UN report to officially lay blame for the attack on Damascus, the UN Commission of Inquiry (COI) on Syria said it had gathered an "extensive body of information" showing Damascus was behind the horrific sarin gas attack in Khan Sheikhun on April 4 that killed at least 83 people