The Organization for the Prohibition of Chemical Weapons (OPCW) has concluded that mustard gas was used during a late-August attack on the Syrian town of Marea. The chemical weapon was detected during a battle between Islamic State insurgents and rebel fighters just north of the ISIS stronghold in Aleppo. OPCW’s confidential report (the media was given a peek at a summary) shows that at least two people were exposed to “sulfur mustard.”

What officials don’t know is which side unleashed it.

Via Reuters:

“It is very likely that the effects of sulfur mustard resulted in the death of a baby,” it said.

The findings provide the first official confirmation of use of sulfur mustard, commonly known as mustard gas, in Syria since it agreed to destroy its chemical weapons stockpile, which included sulfur mustard.

The report did not mention Islamic State, as the fact-finding mission was not mandated to assign blame, but diplomatic sources said the chemical had been used in the clashes between Islamic State and another rebel group taking place in the town at the time.

“It raises the major question of where the sulfur mustard came from,” one source said. “Either they (IS) gained the ability to make it themselves, or it may have come from an undeclared stockpile overtaken by IS. Both are worrying options.”

It’s a fair question. Syria supposedly rid itself of its entire chemical weapons program back in September of 2013 amid uproar following the deaths of hundreds of people in a sarin gas attack outside of Damascus. The purging was to have been completed over a year ago—but this is Assad we’re talking about. International officials and activists have long suspected that Assad squirreled away part of his chemical weapons stockpile, which contained large amounts of mustard gas.

According to Reuters, the Syrian government has accused insurgent forces of using chemical weapons against the military, but those reports remain unsubstantiated.

Meanwhile, the US and allied forces have agreed to increase aid to rebel forces countering the influence of Assad’s Russian- and Iranian-backed push to power. The aid will include shipments of weapons and other supplies.

More from WSJ:

The deliveries from the Central Intelligence Agency, Saudi Arabia and other allied spy services deepen the fight between the forces battling in Syria, despite President Barack Obama’s public pledge to not let the conflict become a U.S.-Russia proxy war.

U.S. officials said the Obama administration is pursuing what amounts to a dual-track strategy, which aims to maintain military pressure on Mr. Assad and his Russian and Iranian supporters while U.S. diplomats see if they can ease him from power through negotiations. U.S. officials said the pressure track was meant to complement the diplomatic track by giving the U.S. leverage at the negotiating table.

Saudi and Turkish officials say the level of U.S. support for rebel groups remains insufficient, despite the latest U.S. promise to do more. Pledges to expand the weapons pipeline came as Washington sought support from Saudi Arabia, Turkey and other allies in the region for a new diplomatic track that includes Russia and Iran, diplomats in the region said.

The US has made a point to emphasize the divide between Assad’s trifecta, and the rest of the world. Officials say that their goal is to direct all of their efforts into defeating ISIS, and not into a proxy war between the US and Russia:

US officials have rejected demands from both the Saudis and the Turks to keep the flow of weapons coming; the Saudis have also repeatedly asked that the US not back down from demands that Assad leave office at the end of a transition period.

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