U.S. military officials in Iraq are walking back claims that American and Iraqi forces were hit with a chemical attack by the Islamic State, telling reporters Friday that evidence of such an attack remains inconclusive.

Initial tests of two mortar rounds that struck near U.S. positions in the vicinity of a military airfield in the western part of al Qayyara, about 40 miles south of Mosul, in northern Iraq on Tuesday showed evidence of a chemical agent akin to mustard gas, Col. John Dorrian, the top U.S. military spokesman in Iraq, said Friday.

The airbase has been designated by Iraqi and American commanders as the main logistics hub to support a series of firebases that will serve as jump-off points for 14 Iraqi Army brigades and thousands of peshmerga and militia fighters for the eventual siege of Mosul, the de facto Iraqi capital for Islamic State, also known as ISIS, ISIL and Daesh.

However, subsequent tests for mustard gas agents on one of the the two mortar rounds proved inconclusive, Col. Dorrian told reporters at the Pentagon during a briefing from Baghdad.

U.S. military officials are awaiting the results of a third round of testing on the artillery shell, he said. If confirmed, it will have been the first use of mustard gas against U.S. troops since World War I.

But Col. Dorrian noted that until the findings of the latest round of tests are finalized, there remains “no conclusive evidence” a chemical attack was carried out in Qayyara.

Col. Dorrian’s comments come a day after Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford testified before Congress that Iraqi and U.S. positions had been hit with a chemical strike.

The four-star general told members of the Senate Armed Services Committee Thursday that a “mustard blister agent” was released near American and allied positions near the Qayyara airfield.

For his part, Gen. Dunford said the presence of the chemical agent at the Qayyara airbase may have been the result of blowback from recent coalition airstrikes against suspected chemical weapons facilities operated by the Islamic State.

“We’ve had a number of strikes … over the past year against [Islamic State’s] emerging chemical capability,” the four-star general said Thursday. “And in this latest strike, again, we assess was a sulfur mustard.”

While there were no reports of any American service members injured as a result of exposure to the gas, Gen. Dunford said Islamic State did possess a “very rudimentary capability to deliver” chemical agents to the battlefield, adding such a capability “was a concerning development.”

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