Pentagon sources tell CBS News that reports are "credible" that the Islamic State of Iraq and Syria (ISIS) likely used mustard gas against Kurdish fighters in Iraq.

It is unclear if the gas was leftover from Saddam Hussein stockpiles or if they were brought over from Syria, which would be the two likeliest sources of the gas, said a senior Department of Defense official. The official said the reports are credible because of prior information, but did not elaborate.

Mustard gas is considered an "antiquated" weapon that must be used in very large concentrations to be lethal, the official told CBS News.

"ISIS is a group that has demonstrated time and again its willingness to stop at nothing," the official said. "This is further evidence of that."

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Earlier Thursday, other officials had said reports were credible that ISIS launched a "crude attack using chemical weapons" against Kurdish fighters. But they had stopped short of identifying the weapon as mustard gas.

A German Defense Ministry statement on Thursday said some 60 Kurdish fighters suffered from breathing difficulties as a result of the attack. It says none of the German soldiers training the Kurds in the area 37 miles southwest of the city of Irbil were hurt or in danger.

Kurdish security official Hiwa Qaraji told The Associated Press that ISIS militants attacked the Kurdish forces, known as peshmerga, near the town of Makhmour late Wednesday, wounding four.

Neither Kurdish nor German officials would specify the type of chemical weapons that may have been used in the attack or provide further details.

CBS News senior national security analyst Juan Zarate said that the attack was significant but not a "game-changer."

"It's not a game-changer with respect to the threat in general," Zarate said. "We've known that ISIS has not only maintained territory and momentum but has been advancing in terms of its weaponry, and there have been concerns that ISIS has gotten its hands on chemical weapons, so that concern is not new.

"The fact that it's being confirmed though, that they used it against peshmerga and the U.S. has to confront it, may be a game-changer, and that may be the real problem here: The reality that ISIS is continuing to expand and adapt and to adapt in ways that are incredibly dangerous."