The Pentagon believes that the Islamic State in Iraq and Syria (ISIS) has used chemical weapons in Iraq, according to CBS.

Officials believe that the weapons were used last week in an attack on Kurdish forces in Iraq and that the terrorist group obtained the weapons in Syria. The Syrian government claims their chemical weapons were destroyed in 2013. However, this claim has never been confirmed by inspectors, according to The Hill. At one point, Syrian chemical weapons reserves included such toxins as sarin and VX.

"We have credible information that the agent used in the attack was mustard," an unnamed senior US official said, according to The Wall Street Journal. Mustard gas is a banned chemical weapon. U.S. intelligence now believes that some stockpiles of chemical agents were hidden from inspectors in Syria. The case is still being investigated.

If ISIS has access to chemical weapons, this could be a devastating turning point in ongoing military actions against this group.

Chemical weapons have been used in modern warfare since World War I, when poisonous gasses were used by both sides of the conflict. The Geneva Protocol, which banned the use of chemical weapons in war, was signed in 1925. In spite of this, chemical weapons continued to be manufactured and were used again in WWII. Since the end of that war, these weapons have only been used a few times. The Chemical Weapons Convention, a disarmament agreement, was adopted in 1992, which was followed by the birth of The Organization for the Prohibition of Chemical Weapons in 1997, according to the UN.