A US military official offered preliminary confirmation that Islamic State (IS) militants used mustard gas during a battle with Kurdish forces earlier this month in Iraq, a development that comes amid reports that the group may have again used chemical weapons last week in Syria.

Marine Corps Brigadier General Kevin Killea, chief of staff of Combined Joint Task Force-Operation Inherent Resolve, the US-led coalition fighting IS, said the results of field tests showed traces of sulfur mustard gas in mortars used by IS in an attack on Kurdish Peshmerga forces on August 11.

The attack occurred in Makhmour, a city in northern Iraq about 40 miles west of Kirkuk. Kurdish forces reportedly saved shell fragments so that the US could test for chemical weapons. German officials training the Kurds reported that at least 60 fighters had difficulty breathing after the attack. Killea said further tests would be done to confirm the preliminary findings.

"It is going to take us a couple of weeks to do the full testing on those fragments to figure out what was contained in or on those mortar rounds before we make a determination on exactly what it was, potentially how much it was and maybe even where it came from," Killea told reporters at the Pentagon on Friday.

The type of mustard gas IS is suspected of using can blister skin and cause lung damage. It is banned under the Chemical Weapons Convention, the international treaty that outlaws chemical weapons and their precursors.

IS was also accused of using chemical weapons on Friday in an attack on the Syrian town of Marea, near Aleppo. According Wall Street Journal, the head of a local hospital reported that people had difficulty breathing, red patches on their skin, watery eyes, and, in severe cases, large pustules on their bodies.

In an interview earlier this month, Olivier Lepick, a research fellow at the Foundation for Strategic Research and an expert in chemical weapons, told VICE News that IS may have acquired technology to put chemicals in rockets or found chemical weapons in Iraq leftover from the Saddam Hussein era.

"IS is obviously interested in carrying out chemical attacks," said Lepick, adding that group had already tried to carry out chlorine attacks. "It's very easy to get your hands on a ton of chlorine and these types of attacks only require limited technical resources."

In 2013, both US and Russian officials were able to pressure the Syrian government to agree to stop using chemical weapons and destroy their stockpile. Some observers have questioned, however, if all those weapons were actually destroyed, or if they might have fallen into the hands of IS.

Killea noted that even if subsequent tests confirm the preliminary findings about the use of mustard gas, little about the fight against IS will change. "We really don't need another reason to hunt down ISIL and kill them wherever we can and whenever we can," he said, using an alternate name for the group.

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