The United States suspects that ISIL radicals have used mustard agent in Iraq, where the militants have ceased vast areas over the past year, The Wall Street Journal reports citing senior US officials.
The banned chemical weapon could have been obtained by ISIL either in Iraq or Syria, another country where the militants have made significant advances, the newspaper said on Thursday.
The theory of the mustard agent originating in Syria "makes the most sense," The Wall Street Journal quoted an anonymous US official as saying.
The US suspects ISIL radicals of using the chemical weapon earlier this week during an attack on Kurdish forces. US intelligence is currently investigating the case.
The Islamic State of Iraq and the Levant (ISIL), a radical Sunni group also known as the Islamic State of Iraq and Greater Syria (ISIS), has been making significant advances in the Middle East, despite the efforts of a US-led anti-ISIL coalition.
The international coalition, formed by US President Barack Obama in September 2014, has been carrying out airstrikes against ISIL targets in both Iraq and Syria.
Obama has also promised to help train and arm Kurds, Iraqis and the so-called "moderate" opposition in Syria, who are fighting against ISIL on the ground. US involvement in ground military operations against ISIL has not been authorized.
According to the Organization for the Prohibition of Chemical Weapons (OPCW), nearly 98 percent of chemical weapons that were removed from Syria after the 2013 sarin gas attack near Damascus have been destroyed.