The joint UN-Organisation for the Prohibition of Chemical Weapons (OPCW) panel said in a much-awaited report that the "sarin was delivered via an aerial bomb that was dropped by an airplane."
The "panel is confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhun on 4 April 2017," said the confidential report to the UN Security Council obtained AFP.
More than 87 people died in the nerve gas attack on the town in Syria's northwestern Idlib province.
Horrific images from the immediate aftermath of the attack drew global outrage and prompted the United States to fire cruise missiles at a Syrian air base from which the West says the assault was launched.
The panel also found that Islamic State militants used mustard gas in an attack on the town of Um Hosh in northern Aleppo region in September 2016.
Syria ally Russia maintains that the sarin attack was most likely caused by a bomb set off directly on the ground, not by a Syrian air strike as alleged by the West.
The report was released two days after Russia vetoed a US-drafted resolution that would have extended by a year the investigation of who is behind chemical weapons attacks in Syria.
Russia said it wanted to study the report by the panel known as the Joint Investigative Mechanism (JIM) before deciding on the extension.
US Ambassador Nikki Haley welcomed the findings and said the Security Council must send a "clear message" that the use of chemical weapons will not be tolerated.
"Ignoring the overwhelming amount of evidence in this case shows a purposeful disregard for widely agreed international norms," Haley said in a statement.
"The Security Council must send a clear message that the use of chemical weapons by anyone will not be tolerated, and must fully support the work of the impartial investigators."
"Countries that fail to do so are no better than the dictators or terrorists who use these terrible weapons,” she added.
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