Suspicions that the deadly gas had been used last August emerged after nearly three dozen Kurdish forces became ill during battle.

The OPCW confirmed on Monday that laboratory tests had come back positive for the sulfur mustard.

The OPCW didn't identify who used the chemical agent, but an unnamed diplomat familiar with the testing confirmed that chemical weapons had been used by "Islamic State" ("IS") fighters.

The battle occurred southwest or Irbil, the capital of Iraq's Kurdish region. Kurdish soldiers became ill during the fight against IS, and dirt samples were taken shortly thereafter.

Mustard gas used in Syria

In October the OPCW concluded that mustard gas had been used earlier in the year in Syria. IS territory spans the Iraq-Syrian border.

Where the gas actually came from is unclear. Hussein essentially destroyed Iraq's arsenal even before the US threatened to invade in 2003. Subsequently, US troops scoured the country but only found old Saddam-era chemical munitions.

Syria is also supposed to have surrendered its stockpile of chemical weapons in 2014, under international supervision.

Syria was forced to give up its chemical weapons, including reserves of sulfur mustard after President Bashar Al-Assad was accused by Western countries of killing hundreds of civilians with sarin nerve gas in a Damascus suburb in 2013.

bik/kms (Reuters, AFP)