Isis militants attacked Kurdish forces in Iraq with mustard gas last year, the first known use of chemical weapons in Iraq since the fall of Saddam Hussein, according to one Western official.
A source at the Organisation for the Prohibition of Chemical Weapons (OPCW) told Reuters that laboratory tests had come back positive for the sulfur mustard, after around 35 Kurdish troops were taken sick on the battlefield last August.
The OPCW would not identify who used the chemical agent. But the official, speaking on condition of anonymity because the findings have not yet been released, said the result confirmed that chemical weapons had been used by Isis fighters.
The OPWC, an intergovernmental organisation based in the Netherlands, has the task of monitoring adherence to the Chemical Weapons Convention, an agreement which prohibits the use of chemical weapons and requires their destruction. No one from the OPWC immediately responded to inquiries on Monday.
However, the news agency said that the samples that tested positive for mustard gas, were taken from soldiers who became after fighting against Isis militants near Erbil, the capital of Iraq's autonomous Kurdish region.
The OPCW had already concluded last October that mustard gas was used last year in neighboring Syria. Islamic State has declared a "caliphate" in territory it controls in both Iraq and Syria and does not recognise the frontier.
Experts believe that the sulfur mustard either originated from an undeclared Syrian chemical stockpile, or that militants have gained the basic know how to develop and conduct a crude chemical attack with rockets or mortars, Reuters said.
Iraq's chemical arsenal was mainly destroyed in the Saddam era, although US troops encountered some old Saddam-era chemical munitions during the 2003-2011 US occupation.
Saddam Hussein used chemicals weapons - among them mustard gas - on several occasions during his time as Iraq’s ruler. In March 1988, up to 5,000 people were killed and thousands more injured, after Iraqi forces dropped chemical munitions on the Kurdish city of Halabja.
Saddam also used chemical weapons on a number of occasions during the 1980-88 Iran-Iraq war, much of it made up of mustard gas, a blistering agent that destroys the eyes, skin and respiratory tract. It was used with devastating effect during World War One.
Syria gave up its own chemical weapons, including stockpiles of sulfur mustard, under international supervision after hundreds of civilians were killed with sarin nerve gas in a Damascus suburb in 2013. Western countries blame President Bashar al-Assad for the attack, though his government denies it.