The Kurdish region's Security Council said in a statement to Reuters that the peshmerga had taken soil and clothing samples after an Islamic State suicide bombing in northern Iraq in January. It said laboratory analysis showed "the samples contained levels of chlorine that suggested the substance was used in weaponised form."
Chlorine is a choking agent whose use as a chemical weapon dates back to World War One. It is banned under the 1997 Chemical Weapons Convention.
It was not possible to independently verify the Kurdish allegation.
The statement said the analysis was carried out in a European Union-certified laboratory after the soil and samples were sent by the Kurdish Regional Government to a "partner nation" in the U.S.-led coalition that is fighting Islamic State militants in Iraq and Syria. A source in the Kurdish Security Council declined to identify the laboratory.
The Jan. 23 suicide car bombing took place on a highway between Mosul and the Syrian border where peshmerga forces were preparing defensive positions after a two-day offensive, the statement said.
The Kurdish source said that the peshmerga fired a rocket at the car carrying the bomb so there were no casualties from the incident, because it exploded before reaching its target.
The BBC also reported that Iraqi officials have video footage, which they say proves Islamic State militants are using chlorine gas in roadside bomb attacks. The videos show bomb disposal teams carrying out controlled explosions, which send plumes of orange smoke into the air.
There have been multiple reports that IS has been deploying chlorine gas since late last year, but Iraqi officials say their footage confirms its use.
"They have resorted to this new method," Haider Taher, from the Iraq Bomb Disposal Team, told the BBC. "They're putting chlorine inside these homemade roadside bombs, which is toxic for those that inhale it."
He said Iraqi troops have defused dozens of devices containing chlorine as part of the offensive against the militants.
Meanwhile, the United States and its coalition partners staged 10 air strikes on Islamic State militants in Iraq and Syria in a 24-hour period, the Combined Joint Task Force said on Saturday.
The strikes, which began on Friday, hit a fighting position and tactical units near the Syrian cities of Kobani and Al Hasakah, the task force said in a statement on the latest daily raids.
In Iraq, four strikes targeted a large Islamic State unit, two tactical units, four buildings and vehicles near Kirkuk. Four other attacks near Mosul, Falluja and Rawah hit a tactical unit, vehicles and a structure. (Reporting by Isabel Coles; Writing by Maggie Fick; Editing by Mark Trevelyan) (In Washington, writing by Bill Trott Editing by Alison Williams)