THE HAGUE, Netherlands (AP) — Insurgents in Syria used sulfur mustard in August, likely killing an infant, the Organization for the Prohibition of Chemical Weapons said Friday.

The OPCW statement was the first official confirmation that sulfur mustard, often called mustard gas, has been used by insurgents in Syria.

An OPCW source told The Associated Press on Thursday that inspectors found evidence of sulfur mustard use by insurgents in the town of Marea on Aug. 21. Friday's official confirmation shows another worrying escalation in Syria's brutal civil war, which has already killed 250,000 people and forced millions to flee.

"In this case, the team was able to confirm with utmost confidence that at least two people were exposed to sulfur mustard, and that it is very likely that the effects of this chemical weapon resulted in the death of an infant," the OPCW said.

The inspectors based their conclusions on samples they analyzed and interviews with two survivors and the physicians who treated them.

Sulfur mustard is an outlawed chemical weapon that attacks the skin, eyes, lungs and other internal organs.

When Syria joined the OPCW in 2013 under pressure from the international community, it declared a 1,300-ton chemical weapon arsenal that has since been destroyed. It is not clear how insurgents came to possess sulfur mustard.

The international watchdog also said another mission investigating allegations that toxic chemicals were used as weapons in Syria found that chlorine likely was used in March this year in fighting in Idlib.

In a third probe, this one into claims by the Syrian government that its troops were targeted with toxic chemicals in Jobar on Aug. 29, inspectors said they "could not confidently determine that a chemical was used as a weapon."

The findings, which do not apportion blame for the chemical attacks, will be sent to the United Nations Security Council.