Sarin nerve agent was used in an 'incident' at a northern Syrian village in late March, five days before the deadly attack on Khan Sheikhun, the world's chemical watchdog said Wednesday.
"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 Organisation for the Prohibition of Chemical Weapons told AFP in an interview.
"The results prove the existence of sarin," Ahmet Uzumcu said.
The Khan Sheikhun attack on April 4 was previously believed to have been the first use of sarin since the deadly August 2013 attack in and around Damascus which killed hundreds of people.
But Uzumcu said sarin was used in the opposition-held village of Latamneh, some 25 kilometres (15 miles) south of Khan Sheikhun on March 30.
"What we know at the moment is not much. Fifty people were reportedly injured. There were no deaths reported," he said.
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.
- Troubling new information -
At the United Nations in New York, the Security Council met behind closed doors to discuss chemical weapons use in Syria and ongoing investigations by the JIM, the joint OPCW-UN panel tasked with identifying perpetrators of attacks.
French Ambassador Francois Delattre said the council was awaiting details of the Latamneh attack, but said the new information was "very troubling to say the least".
France and Britain said the JIM, which is due to submit its report on the Khan Sheikhun attack later this month, must be allowed to continue its work for another year amid fears that Russia may try to shut it down.
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Two days after Khan Sheikhun, 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 Sheikhun, in the opposition-held province of Idlib.
Uzumcu pointed out that the OPCW's fact-finding mission team was unlikely to visit the area, where fighting is still ongoing between Syrian government forces and armed opposition groups.
"The (fact-finding team) is making every effort to contact the victims," Uzumcu said.
- Damascus denies -
Syria's government has denied involvement and claims it no longer possesses chemical weapons after a 2013 agreement under which it pledged to surrender its chemical arsenal.
The OPCW earlier this year presented a report confirming sarin gas was used in the attack at Khan Sheikhun, but did not assign blame.
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 President Bashar al-Assad's regime.
Damascus has vehemently refuted the claims saying "Syria has not and will not use toxic gases against its people because it does not have them."
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 government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that Islamic State jihadists used mustard gas in 2015.
Uzumcu told AFP that if the JIM is able to identify those responsible for the Khan Sheikhun attack, "they should certainly be held accountable, prosecuted and punished."
"That's the only way to keep strong the international norm against the use of chemical weapons," he said.