|Dozens of children were hurt in the gas attack|
The Organisation for the Prohibition of Chemical Weapons report into the Khan Sheikhoun attack has not been publicly released but two diplomats familiar with it confirmed the key finding.
UK Foreign Secretary Boris Johnson said he was "pleased though, I must say, not surprised" that the report had confirmed the use of sarin.
OPCW director general Ahmet Uzumcu had said two weeks after the attack that samples from victims and survivors had shown they were exposed to sarin or a sarin-like substance.
|'First step' in holding Assad to account|
The report, while establishing chemical weapons were used, did not name the perpetrator.
Mr Johnson said the condemnation of chemical weapons use was having a "deterrent effect".
He added: "What's most interesting and important of all is that you're starting to see Russian impatience with... Assad, a recognition that he really embarrassed Moscow by allowing his generals to do this and that is one of the objectives that we're trying to achieve: to drive a wedge between the Russians and the Iranians and between the Russians and Assad."
US envoy to the United Nations Nikki Haley said in a statement: "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."
The attack in April caused an international uproar, with photos seen around the world of children struggling for breath and dying.
US President Donald Trump cited the attack when he launched an airstrike on Syria a few days later , firing cruise missiles on a Syrian air base from where the US said the attack had been launched.
Syria's President Bashar al Assad has denied responsibility for the attack.
When the Syrian government joined the OPCW in 2013, after it was blamed for a poison gas attack in Damascus, it declared around 1,300 tons of chemical weapons which were later destroyed.
But there have been doubts raised over whether the country's initial declaration was true.