WASHINGTON — Islamic State militants have used mustard agents to attack opponents in Syria in recent weeks, a U.S. official said Friday, stoking the Obama administration’s suspicions that the extremists used them on Iraqi Kurdish forces as well.

“We find the allegations of mustard use there (in Iraq) to be plausible because of information we already had – that they certainly had it in their possession, and used it in the past,” a U.S. official said, referring to the Islamic State. Like others, the official spoke on the condition of anonymity to discuss an internal assessment.

The Obama administration believes the extremist group’s mustard attack took place in Syria earlier this month, but did not provide additional details.

Islamic State militants also used what is suspected to have been a chemical weapon in an attack on Kurdish fighters in Iraq this week.

Canisters of poisonous gas were fired at Kurdish forces on a frontline in Makhmour, around 25 miles southwest of Irbil on Tuesday, but did not cause any fatalities, said Falah Mustafa Bakir, the minister of foreign affairs for Iraq’s Kurdish region.

“It’s not the first time ISIS has used mortars with something in it,” Bakir said. However, this time the Kurdish regional government appealed to its partners in the U.S.-led coalition to analyze samples, he said, adding that the United States, Germany and France are all involved in the investigation. “We are still waiting for the results; there has been some talk about chlorine, some about mustard gas,” he said.