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.