THE HAGUE, Netherlands - The U.S. envoy to the international chemical weapons watchdog warned Monday that the use of such toxic arms is "becoming routine in the Syrian civil war."

Rafael Foley was speaking at a closed meeting of the Organization for the Prohibition of Chemical Weapons' Executive Council, which was called to discuss recent reports by a fact-finding mission, including that a "non-state actor" likely used the chemical agent sulfur mustard in August during fighting in the Syrian town of Marea, killing a baby.

Foley said Syrian opposition forces were fighting the Islamic State of Iraq and Syria (ISIS) in the town close to the Turkish border. The text of his speech was posted on The Hague-based watchdog's website.

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The fact-finding mission, which took and tested samples and interviewed witnesses, also said that chlorine likely was used as a weapon between March and May in Idlib, leaving six people dead.

After the meeting, attended by representatives of 38 member states, the executive council issued a statement "expressing grave concern" about the fact-finding mission's conclusions that "chemical weapons have once again been used in" Syria and saying that those responsible should be held accountable.

A special investigation team has been set up by the United Nations and OPCW to identify who is responsible for chemical attacks in Syria.

French Prime Minister Manuel Valls warned last week that associates of extremists responsible for the Nov. 13 Paris attacks could use chemical and biological weapons.

The U.S. has repeatedly accused the Assad regime in Syria of purposefully using chemical weapons.

In December of 2014, Undersecretary of State Rose Gottemoeller told members of the Organization for the Prohibition of Chemical Weapons: "Now there is compelling evidence that Syria continues to use chemical weapons systematically and repeatedly."