Mustard gas chemical weapon
Representational Image: ISIS used mustard gas against Kurdish fighters in Iraq in 2015, tests have shown. In picture: Mustard gas-filled 105mm projectiles are seen inside a chemical storage igloo, at the Pueblo Chemical Depot in an undated photo provided by the U.S. Army Chemical Materials Activity (CMA) in Pueblo, Colorado.Reuters

The Islamic State group used mustard gas as a chemical weapon to attack Kurdish forces in Iraq last year, tests by the Organisation for the Prohibition of Chemical Weapons (OPCW) reportedly show. This is said to be the first time since the fall of former Iraqi ruler Saddam Hussein that the chemical agent was used in the country. 

Kurdish forces, also known locally as the Peshmergas, have been putting up a stiff fight against the ISIS militants in Iraq, and some of the troops came under attack from mustard gas in August 2015 in Erbil, the capital of the autonomous Kurdish region in Iraq. At least 35 Kurdish fighters had fallen ill, and tested positive for the sulphur mustard, the OPCW told Reuters

Sulphur mustard causes severe burns to the eyes and skin and also affects the respiratory tract.

While a diplomat told the news agency that this was the first known use of chemical weapons in Iraq since the Saddam Hussein-era, local media had been reporting of attacks with chemical agents by ISIS since 2014. 

Chemical warfare expert Hamish de Bretton-Gordon, who had earlier warned of the Islamic State group's ability to use chemical weapons due to access to stockpiles of mustard gas and sarin gas in Iraq, said that the mustard gas agent were being produced in the ISIS-stronghold of Mosul. 

"I'm pretty convinced that the mustard ISIS are using in Iraq is made by them in Mosul. They have all the precursors at hand from the oil industry and all the experts at hand to do it," he told Reuters. 

Last week, U.S. Director of National Intelligence James Clapper said that the use of chemical weapons by ISIS was the first time an extremist group was producing and using "a chemical warfare agent in an attack, since Aum Shinrikyo used sarin in Japan in 1995." The Japanese extremist group had used sarin gas to attack the Tokyo subway in 1995.

The OPCW had also pointed at the use of mustard gas by ISIS in Syria last year. 

U.S. Central Intelligence Agency (CIA) Director John Brennan warned that ISIS was making small quantities of chlorine and mustard gas for chemical weapons that the group could also export to the West.