"The IIT is in charge of identifying the perpetrators and in the next few months we are going to be in the position to produce the first report."
The OPCW has said its previous analysis provided reasonable grounds that toxic chemicals had been used in attacks in Syria.
The watchdog did not say who was responsible. The ITT, set up by the OPCW in 2018, got to work in June to assign blame for the attacks.
Arias was at the United Nations headquarters to brief the Security Council on his organization's work in Syria.
He said two main issues were to "verify that Syria has fully declared its entire chemical weapons stockpiles," and to "investigate allegations of the use of toxic chemicals as weapons in Syria since 2013," when it joined the Chemical Weapons Convention.
There have been numerous attacks involving toxic weapons in the war-torn country since then, for which both the Syrian government and rebel forces have been blamed.
In March, the UN Human Rights Council blamed Syrian President Bashar al-Assad's government for 32 of the 37 publicly reported instances of the use of chemical weapons in Syria.
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