A UN fact-finding mission has discovered that a number of people in Syria may have been exposed to sarin, a toxic chemical weapon, or a sarin-like gas. It reported its findings on Monday.

The mission, conducted by the Hague-based Organization for the Prohibition of Chemical Weapons (OPCW), investigated 11 incidents of the use of toxic chemicals reported by the Syrian government. It did not specify when or where the incidents took place.

The report found that “In one instance, the analysis of some blood samples indicates that individuals were at some point exposed to sarin or a sarin-like substance”, and that “further investigation would be necessary to determine when or under what circumstances such exposure might have occurred”.

The UN Security Council is set to discuss Syria’s usage of chemical weapons at a meeting on Tuesday.

During previous fact-finding missions in Syria, the OPCW has reported the use of chlorine and mustard gas against Syrian rebels and citizens by the government, though Syria has denied ever using chemical weapons in the ongoing civil war.

Throughout the conflict, both President Bashar Assad and the leaders of the rebel forces have accused each other of using chemical agents.

After a sarin attack outside Damascus in 2013, the blame for which the international community placed on Assad’s regime, the Syrian government agreed to turn over its chemical arsenal.

According to the OPCW report, 99.6 percent of all declared chemical weapons in Syria have been destroyed.