The State Department on Wednesday said it believes the Islamic State in Iraq and Syria (ISIS) used sulfur mustard chemical weapons in one specific instance in Syria, as well as in Iraq, last year.
"We believe that the Islamic State for Iraq and the Levant, ISIL, or Daesh, was responsible for a sulfur mustard attack in Marea in Aug. 21, 2015," State Department Deputy spokesman Mark Toner said during a briefing, using administration's preferred name for the terrorist group.
It was the first time a U.S. official has publicly confirmed that the administration believes ISIS was behind the Aug. 21 attack in Syria.
Toner said that determination was "largely based on photographic evidence as well as Syrian opposition's description of the event."
Toner also said the U.S. believes that ISIS was "likely responsible" for some of the alleged attacks using sulfur mustard in Iraq.
Two U.S. defense officials recently confirmed to The Hill that ISIS indeed used chemical weapons in one attack in August against the Kurdish peshmerga.
After preliminary field tests showed the presence of mustard agent, the U.S. undertook fuller testing of mortar fragments to determine what the substance was, how much was present and where it may have came from.
While the results have not been released, one U.S. defense official said they showed ISIS's rudimentary ability to manufacture chemical weapons with chlorine and mustard agent.
The State Department's public acknowledgment comes after James Clapper, the director of National Intelligence, said last week that ISIS has used chemical weapons in Iraq and Syria and after CIA head John Brennan said ISIS has the ability to make and use such weapons.
Defense Secretary Ashton Carter said on PBS's "Charlie Rose" on Tuesday that the U.S. is watching ISIS's chemical weapons capability and that it is 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.