Defense Secretary Ashton Carter said late Tuesday that the U.S. watches the Islamic State in Iraq and Syria's (ISIS) emergent chemical weapons capability "very closely" and that it's something the U.S. takes "action" against.
"I'm just telling you it's something we watch very closely and it's something we take action against," Carter said on PBS's "Charlie Rose."
Carter also indicated the coalition has struck "wherever we've seen that kind of ... indication of interest."
The U.S. last year killed ISIS chemical weapons expert Abu Malik, who was a chemical weapons engineer at Saddam Hussein's Muthana chemical weapons production facility.
However, it's not clear what else the U.S.-led coalition is doing to stop ISIS's chemical weapons capability from growing.
ISIS has used chemical weapons against Kurdish peshmerga at least a handful of times in Iraq, according to the Kurdistan Regional Government.
Two U.S. defense officials recently confirmed to The Hill that lab testing showed at least one instance where ISIS used chemical weapons against the peshmerga in northern Iraq last August.
There have been reports that ISIS has used chemical weapons in Syria, too.
Testing of about 10 samples showed that ISIS had the ability to make rudimentary chemical weapons using mustard agent and chlorine, a U.S. defense official told The Hill.
Carter suggested that ISIS has obtained the components from facilities in Iraq.
"ISIL occupies a considerable part of Iraqi territory and, therefore, there are people there and there are industrial facilities there that could contribute to — and depends on what you mean by chemical weapons here — but chemistry that could be used for harmful purposes there," he said, using another acronym for ISIS.
"We have seen evidence of that from time to time," he added. "But remember, ISIL does lots of really nasty things, brutal things and this is a group we have to defeat and quickly for all these reasons," he said, using the administration's preferred acronym for the terrorist group.
House Majority Leader Kevin McCarthy (R-Calif.) blasted the administration for not doing more to prevent ISIS from gaining the capability.
"ISIS has remained resistant to this Administration’s halfhearted attempts to degrade and destroy it. In the time ISIS has had to solidify its position, the terrorist organization has developed chemical weapons, including Sulfur mustard, a blister agent, and mustard gas," he said in a statement Tuesday.
"This proliferation and increased use of chemical weapons is dangerous and violates all standards of humanity. The longer we allow ISIS to exist, the more ISIS can develop these weapons and use them against our allies or potentially even American troops in the region," he said.