Sarin nerve agent was used in an 'incident' at a northern Syrian village in late March, five days before the deadly attack on Khan Sheikhoun that left more than 80 people dead, the world's chemical watchdog has said.
"Analysis of samples collected (by the OPCW)... relates to an incident that took place again in the northern part of Syria on the 30th of March this year," the head of the Organization for the Prohibition of Chemical Weapons told AFP in an interview on Wednesday.
"The results prove the existence of sarin," Ahmet Uzumcu said.
The Khan Sheikhoun attack on April 4 was previously believed to have been the first use of sarin by the Syrian regime since the deadly August 2013 attack in and around Damascus which killed hundreds of people.
Two days after Khan Sheikhoun, the United States fired 59 Tomahawk missiles at a Syrian airbase from which it said the attack was launched.
At least 87 people including 30 children died in the attack on Khan Sheikhoun, in the opposition-held province of Idlib.
But Uzumcu said sarin was used in the opposition-held village of Latamneh, some 25 kilometers south of Khan Sheikhun on March 30.
He said the OPCW's fact-finding mission had retrieved soil samples, clothing and metal parts "which were sent to our laboratories and we received the results a few days ago".
It is "worrying that there is some sarin use or exposure even before the April 4 incident," he said.
Uzumcu pointed out that the OPCW's fact-finding mission team was unlikely to visit the area, where fighting is still ongoing between Syrian regime forces and armed opposition groups.
But “the (fact-finding team) is making every effort to contact the victims," Uzumcu said.
Syria's regime has denied involvement and claims it no longer possesses chemical weapons after a 2013 agreement under which it pledged to surrender its chemical arsenal.
It says "Syria has not and will not use toxic gases against its people because it does not have them."
UN war crimes investigators last month said they had evidence that Syrian forces were behind the attacks, the first UN report to officially blame the Bashar al-Assad regime.
In total, the OPCW is investigating as many as 45 suspected chemical attacks in Syria since mid-2016, the watchdog said in April.
The JIM has already determined that Syrian regime forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that ISIS militants used mustard gas in 2015.