A banned nerve agent sarin was used in an attack in northern Syria in April, said the world's chemical weapons watchdog, a report from a fact-finding team seen by Reuters on Thursday showed.
The attack on April 4 in the town of Khan Sheikhoun in northern Idlib province that killed more than 80 people and wounded at least 550 others was the deadliest in the Syrian war in more than three years.
After interviewing witnesses and examining samples, a fact-finding mission of the Organisation for the Prohibition of Chemical Weapons (OPCW) concluded that "a large number of people, some of whom died, were exposed to sarin or a sarin-like substance."
"It is the conclusion of the FFM [fact-finding mission] that such a release can only be determined as the use of sarin, as a chemical weapon," a summary of the report said.
"Now that we know the undeniable truth, we look forward to an independent investigation to confirm exactly who was responsible for these brutal attacks so we can find justice for the victims," US Ambassador to the UN Nikki Haley said in a statement on Thursday.
The report was circulated to members of the OPCW in The Hague, but was not made public.
The attack prompted a US missile strike against a Syrian air base which Washington said was used to launch the strike.
A joint UN and OPCW investigation, known as the JIM, can now look at the incident to determine who is to blame, Nikki Haley said.
The JIM has found Syrian regime forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Daesh used mustard gas.
Western intelligence agencies had also blamed the regime of Bashar al Assad for the attack.
But Syrian officials have repeatedly denied using banned toxins in the conflict.
The mission was unable to visit the site itself due to security concerns and will not attempt to get there, the head of the OPCW was said to have decided.
Syria joined the chemicals weapons convention in 2013 under a Russian-US agreement, averting military intervention under then-US president Barack Obama.
The US said on Wednesday the Syrian regime appeared to have heeded a warning this week from Washington not to carry out a chemical weapons attack.
Russia warned it would respond proportionately if the US took preemptive measures against Syrian forces after Washington said on Monday it appeared the Syrian military was preparing to conduct a chemical weapons attack.