REPORT: US officials believe ISIS may have used mustard agent in Iraq

WASHINGTON (Reuters) - The United States believes Islamic State militants likely used mustard agent in an attack on Kurdish forces in Iraq earlier this week, the first indication the militant group has obtained a banned chemical weapon, the Wall Street Journal reported on Thursday.

Citing senior U.S. officials, the Journal said Islamic State could have obtained the mustard agent in Syria, whose government admitted to having large quantities of the blistering agent in 2013, when it agreed to give up its chemical weapons arsenal.

Islamic State could also have obtained the mustard agent in Iraq, the Journal reported.

Inspectors have faced siginificant challenges overseeing the removal of chemical agents from Syria. As reported by Business Insider's Natasha Bertrand last month, many of the production facilities were hidden in plain sight.

As part of an agreement reached in 2013 Syria's stockpile of chemical weapons were to be destroyed at sea, but as recently as June this year, international inspectors found traces of chemical weapons "at sites the Assad regime had failed to report to the Organization for the Prohibition of Chemical Weapons."

The Obama administration reportedly knew about those undeclared sites.

In a statement to the Associated Press, Alistair Baskey with the White House National Security Council says the US takes the allegations "very seriously" and is hunting down more information.

(Reuters reporting and editing by Eric Beech and Peter Cooney)