The Syrian government of Bashar Assad is to blame for a chemical attack on the opposition-held town of Khan Sheikhoun that killed dozens of people last April, according to a report sent to the United Nations Security Council on Thursday.

"The Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhoun on 4 April 2017," the report from the U.N. and the Organization for the Prohibition of Chemical Weapons' Joint Investigative Mechanism (JIM) said.

The attack prompted a U.S. missile strike against a Syrian airbase that Washington said was used to launch the strike.