U.S. officials believe the terror group ISIS used chemical weapons in Iraq this week, defense and intelligence officials said Thursday.

The terror group used mustard gas in an attack against Kurdish fighters, two defense officials and an intelligence official told NBC News. Mustard gas, infamously used during World War I, can cause blisters, blindness and respiratory damage.

The defense officials said this is the second time ISIS has used chemical weapons — the first occurred in "recent weeks" in Syria — but they declined to provide more details, citing operational security concerns.

Kurdish fighters have reported that chemical weapons had been used in ISIS attacks in the past. In March, the Kurdistan Region Security Council claimed that chlorine gas was used in a Jan. 23 suicide truck bomb attack in northern Iraq. Thursday was the first time U.S. officials have said they believe chemical weapons were used by ISIS.

This week, the U.S. launched its first manned military flights against ISIS from an airbase in Turkey. Turkey last week decided to host American aircraft supporting Free Syrian Forces, shortening the distance for pilots conducting airstrikes in Syria.

The U.S. began airstrikes against ISIS in Syria and Iraq a little more than a year ago, and unmanned U.S. drones have been launched from inside Turkey into Syria. But manned aircraft previously have had to take off from the Persian Gulf, hundreds of miles away from targets in Syria.