A fact-finding mission by the UN's chemical watchdog, the OPCW, concluded that sarin was used as a chemical weapon in the April 4 attack in the Syrian town of Khan Sheikhun, a confidential report said Thursday.

The findings by the Organization for the Prohibition of Chemical Weapons (OPCW) will now be taken up by a joint UN-OPCW panel to determine whether Syrian government forces were behind the attack.

"Based on its work, the FFM (fact-finding mission) is able to conclude that a large number of people, some of whom died, were exposed to sarin or a sarin-like substance," said the report, parts of which were obtained by AFP.

"The release that caused this exposure was most likely initiated at the site where there is now a crater in the road," it added.

"It is the conclusion of the FFM that such a release can only be determined as the use of sarin, as a chemical weapon."

At least 87 people including many children were killed in the attack that the United States, France and Britain have said was carried out by President Bashar al-Assad's forces.

The United States launched a retaliatory cruise missile strike days later against a Syrian airbase from where it said the chemical weapons attack was launched.

US Ambassador Nikki Haley said in a statement that she had the "highest confidence in the OPCW report" which delivered its final conclusion about the sarin gas attack.

"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," she added.

The OPCW-UN joint investigative mechanism (JIM) has already determined that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that the Islamic State used mustard gas in 2015.

Russia, Syria's ally, has dismissed the findings as not credible. In February, Moscow vetoed a UN resolution that would have imposed sanctions on Syria over chemical weapons use in the six-year war. 

Source: AFP