The Organisation for the Prohibition of Chemical Weapons on Monday voiced "serious concern" over reports that the Islamic State group has used chemical weapons in Iraq.

"Recent reports of possible use of chemical weapons in Iraq by non-State actors are a matter of serious concern," The Hague-based OPCW said in a statement.

German officials said last week that Kurdish fighters had been attacked on Tuesday with chemical weapons, possibly chlorine or mustard gas, in northern Iraq, leaving many peshmerga suffering from "respiratory irritation.

The allegations, deemed "plausible" by a US official, follow claims in March by the autonomous Kurdish government in northern Iraq which said it had evidence that the jihadist group used chlorine in a car bomb attack on January 23.

The Wall Street Journal cited US officials as saying they believe last week's attack used mustard gas, which may have come from stockpiles of banned poisons that Syrian President Bashar al-Assad was forced to get rid of after joining the OPCW in 2013.

The OPCW said it was in contact with the Iraqi government and "will examine any substantive reports it receives".

Along with the January attack, the Conflict Armament Research group and Sahan Research group said last month that IS had also targeted peshmerga with a projectile filled with an unknown chemical agent on June 21 or 22.

The chemical used had characteristics and clinical effects "consistent with a chlorine chemical agent", the groups said.

The organisations said they had also documented two such attacks against Kurdish fighters from the People's Protection Units in Syria's northeastern Hasakeh province on June 28.

It said that upon impact, the projectiles had released a yellow gas "with a strong smell of rotten onions".

There were no deaths but troops exposed to it had experienced burning of the throat, eyes and nose, severe headaches, muscle pain, impaired concentration and mobility, and vomiting.

"Although these chemical attacks appear to be test cases, we expect IS construction skills to advance rapidly as they have for other IEDs (improvised explosive devices)," Emmanuel Deisser, managing director at Sahan Research, said at the time.