The Hague-based chemical weapons watchdog on Monday voiced “grave concern” at the continued use of toxic arms in Syria, calling for those behind such attacks to be held accountable.

A special meeting of the UN-backed Organization for the Prohibition of Chemical Weapons (OPCW) met to discuss the findings of three reports after investigations in Syria.

The body expressed “grave concern regarding the findings… that chemical weapons have once again been used in the Syrian Arab Republic,” the OPCW said in a statement quoted by the AFP news agency.

The use of such weapons by anyone “is unacceptable and would violate international law. Those individuals responsible for the use of chemical weapons should be held accountable,” it added.

The OPCW said several weeks ago confirmed with “utmost confidence” that mustard gas was used in Syria in August during fighting between rebels and jihadists and “likely” killed a child.

That determination followed an earlier announcement that chlorine gas had been used in attacks on three villages in Syria last year.

The OPCW investigations did not directly blame any of the parties in the four-year civil war, aimed at ousting Syrian President Bashar Al-Assad.

But earlier, a U.S. representative to the OPCW, Rafael Foley, said, “The sad reality is that chemical weapons’ use is becoming routine in the Syrian civil war.”

He charged there was only “one conclusion” from the expert reports that “the Syrian regime has continued to use chemical weapons on its own people.”

But accusations have also mounted that extremists with the Islamic State (ISIS) group have resorted to such tactics.

Foley also alleged that even though Syria joined the OPCW in 2013 and declared its stockpile of toxic weapons there were still “discrepancies and omissions.”

This “raises the specter that Syria is maintaining covert stocks of chemical agents as part of a program that is it obligated both to declare and destroy,” Foley told the closed-door meeting.

“The regime’s recent use of chemical weapons against their own people makes abundantly clear that they will not hesitate to resort to such tactics as it suits their cynical purposes,” he added, in his remarks released to reporters.

The European Union also voiced its concerns saying that “the use of chemical weapons by anybody, anywhere and under any circumstances is abhorrent and must be rigorously condemned.”

Under a deal hammered out in 2013 between Russia and the United States following a sarin gas attack on the outskirts of Damascus in which hundreds died, the regime pledged to hand over all its toxic weapons to the OPCW for destruction.

AFP contributed to this report.