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, AFP reported Wednesday (October 4th).

"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 said.

"The results prove the existence of sarin," Ahmet Uzumcu said.

The Khan Sheikun attack on April 4th 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 had been used in a village called al-Lataminah, some 25 kilometres south of Khan Sheikhun on March 30th.

"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 4th incident," he said.

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.

But 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 Assad regime.

The JIM, a joint OPCW-United Nations panel, is now investigating the question of responsibility and its report on the attack is due within the next few weeks.