WATCH: U.S. Investigating Reports ISIS Used Chemical Weapons In Iraq

WASHINGTON — (CNN) The U.S. is investigating what it believes are “credible” reports that ISIS fighters used mustard agent in an attack against Kurdish Peshmerga this week, causing several of them to fall ill, U.S. officials working in at least three separate parts of the Obama administration said Thursday.

All of them strongly emphasized more intelligence is being gathered on exactly what may have happened near the town of Makhmour in northern Iraq. While there have been accounts posted in social media about the incident, the officials said they have independent information that strongly led them to assess there was a use of chemical weapons. The officials would not tell CNN what evidence led them to this belief.

The officials emphasized the intelligence indicates it was likely a small amount of chemical agent and a low concentration. U.S. officials are concerned, but this is not considered to be a massive attack.

A German Ministry of Defense spokesman told CNN it cannot confirm or rule out that there was a chemical weapons attack in the region where German military advisers train Peshmerga.

A senior U.S. official said that ordnance fired at Peshmerga last week in northern Iraq produced a small number of injuries with “wounds consistent with a blister-producing agent.”

The U.S. believes ISIS, also known as ISIL, most likely used either mortar or rocket shells to deliver the chemical warfare agent. One official said those who fell ill had symptoms of breathing problems believed to be associated with mustard gas and not chlorine gas, which is another agent that its believed the Assad regime has used on its civilians in Syria.

Democratic Rep. Adam Schiff of California, ranking member of the House Permanent Select Committee on Intelligence, told CNN’s Wolf Blitzer, “If they do possess these kind of weapons … my guess is they’re more likely to have gotten them as old weapons left over in Iraq from the old WMD program there than they were likely to obtain them in Syria in some kind of a hidden cache of the regime’s.”

A U.S. official said ISIS had used chlorine in the past, but that it is hard to weaponize.

The major question for the U.S. intelligence community now is to determine exactly what happened, and if it is mustard gas, to try to figure out how ISIS came into possession of it. Officials tell CNN it may have come from old chemical weapons caches in Iraq or Syria that the U.S. does not know about.

It’s also not known how much of the agent ISIS may have.

“Did ISIS find some mustard gas shells?” one official asked. “We think they did. We think they have used it.”

According to the Centers for Disease Control and Prevention, mustard gas, also known as sulfur mustard, is a chemical warfare agent. The agent was developed during World War I and was banned by treaty in 1993. While it is usually not fatal, according to the CDC, it can cause blistering of the skin, eye pain and blindness, as well as respiratory problems.

Blake Narenda, a spokesperson for the State Department’s Arms Control, Verification and Compliance Bureau, said, “We continue to take these and all allegations of chemical weapons use very seriously. As in previous instances of alleged ISIL use of chemicals as weapons, we are aware of the reports and are seeking additional information. We continue to monitor these reports closely, and would further stress that use of any chemicals or biological material as a weapon is completely inconsistent with international standards and norms regarding such capabilities.”

CNN has previously reported claims from monitoring groups that ISIS used chlorine weapons against Kurdish forces.