There is evidence that the Islamic State group used chlorine gas as a chemical weapon against peshmerga fighters, Kurdish authorities in Iraq said.
The allegation by the Kurdistan Region Security Council in relation to a suicide truck bomb attack in northern Iraq on January 23 has not yet drawn a reaction from the Islamic State group, which holds a third of Iraq and neighbouring Syria in its self-declared caliphate.
However, Iraqi officials and Kurds fighting in Syria have made similar allegations about the militants using the low-grade chemical weapons against them.
In a statement, the council said the alleged chemical attack took place on a road between Iraq's second-largest city, Mosul, and the Syrian border, as peshmerga forces fought to seize a vital supply line used by the Sunni militants.
It said its fighters later found "around 20 gas canisters" that had been loaded onto the truck involved in the attack.
Video provided by the council showed a truck racing down a road, with white smoke pouring out of it as it came under heavy fire from peshmerga fighters. It later showed a white, billowing cloud after the truck exploded and the remnants of it scattered across a road.
An official with the Kurdish council told reporters that dozens of peshmerga fighters were treated for "dizziness, nausea, vomiting and general weakness" after the attack.
The Kurds say samples of clothing and soil from the site were analysed by an unnamed lab in an unnamed coalition partner nation, which found chlorine traces.
"The fact ISIS relies on such tactics demonstrates it has lost the initiative and is resorting to desperate measures," the Kurdish government said in the statement.
There was no independent confirmation of the claim.
Chlorine, an industrial chemical, was first introduced as a chemical weapon at Ypres in the First World War with disastrous effects as gas masks were not widely available at the time. While chlorine has many industrial and public uses, as a weapon it chokes victims to death.
In the Syrian civil war, a chlorine gas attack on the outskirts of Damascus in 2013 killed hundreds and nearly drove the US to launch airstrikes against the government of embattled President Bashar Assad. The US and Western allies accused Mr Assad's government of being responsible for that attack, while Damascus blamed rebels.
There have been several allegations that the Islamic State group has used chlorine. In October, Iraqi officials claimed Islamic State militants may have used chlorine-filled cylinders during clashes in late September in the towns of Balad and Duluiya.
Their disclosures came as reports from the Syrian border town of Kobani indicated that the extremist group added chlorine to an arsenal that already includes heavy weapons and tanks looted from captured military bases.
Insurgents have used chlorine gas in Iraq before. In May 2007, suicide bombers driving chlorine tankers struck three cities in Anbar province, killing two police officers and forcing about 350 Iraqi civilians and six US troops to seek treatment for gas exposure. Those bombers belonged to al Qaida in Iraq, which later became the Islamic State group.
Meanwhile, Iraqi security forces engaged in fierce clashes with the militants on Saturday as they continued their offensive to retake Saddam Hussein's home town, Tikrit.
Iraqi forces, which include the military, police, Shiite militias and Sunni tribesmen, entered the city of Tikrit for the first time on Thursday, gaining control of neighbourhoods on its northern and southern ends.
Militia commander Hadi al-Amiri has said security forces will hold their position until the area is cleared of any remaining civilians. He estimated on Friday that Iraqi forces would reach the center of Tikrit within two to three days.