By Gordon Lubold and

WASHINGTON—Preliminary U.S. tests show Islamic State fighters used mustard agent against Kurdish Peshmerga fighters in northern Iraq in at least one attack earlier this month, U.S. military officials said Friday.

If confirmed, the use of mustard agent in the Aug. 11 mortar attack on the Kurds in northern Iraq would be the second known instance in which Islamic State has used mustard agent on the battlefield.

U.S. officials said last week that test results confirmed Islamic State used mustard agent against Kurdish forces in northern Syria in late July.

The officials also suspect mustard agent or a similar blistering agent was used in an Aug. 13 attack against the Peshmerga in northern Iraq, but preliminary test results from that incident have yet to be announced.

The preliminary results from the Aug. 11 mortar attack in northern Iraq were based a field test conducted after Peshmerga fighters brought fragments from the mortar rounds used in the attack to U.S. officials for analysis, said Brig. Gen. Kevin Killea, the chief of staff for Combined Joint Task Force Operation Inherent Resolve.

He said those preliminary tests showed the presence of so-called sulfur mustard, which is a banned chemical agent

Gen. Killea said more testing was needed to determine conclusively that Islamic State used mustard agent in the attack, how much of chemical may have been used in the attack and where the chemicals came from.

“That is a presumptive field test, and it is not conclusive,” Gen. Killea said. “What those results tell us is the presence of that chemical, it doesn’t tell us anything more than that.”

Further testing will take another few weeks, Gen. Killea said. But he added that “any indication of the use of a chemical warfare agent, from our perspective, reinforces the position that this is an abhorrent group that will kill indiscriminately without any moral or legal code or restraint.”

U.S. officials say Islamic State militants may have made the mustard agent themselves by combining the chemical ingredients used to make the weapon. Islamic State may also have obtained ready-to-use mustard agent inside Syria, possibly from stockpiles which the Assad regime had controlled.

The attack raised new questions about the evolving threat posed by Islamic State and the ability of U.S. allies on the ground to combat it.

The Wall Street Journal