Kurdish authorities in Iraq said Saturday they have evidence ISIS used chlorine gas as a weapon against peshmerga fighters, the latest alleged atrocity carried out by the extremist organization now under attack in Tikrit.

The allegation by the Kurdistan Region Security Council, stemming from a Jan. 23 suicide truck-bomb attack in northern Iraq, did not immediately draw a reaction from the Islamist jihadists, who hold 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, 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 The Associated Press that dozens of peshmerga fighters were treated for "dizziness, nausea, vomiting and general weakness" after the attack. He spoke on condition of anonymity as he was not authorized to discuss the incident.

Soil, clothes tested

The Kurds say samples of clothing and soil from the site were analyzed 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, using an alternate acronym for the Sunni militant group.

There was no independent confirmation of the Kurds' claim.

Chlorine, an industrial chemical with many peaceful uses, was introduced as a weapon at Ypres in the First World War with disastrous effects as gas masks were not widely available at the time. When used as a weapon it chokes victims to death.

In the Syrian civil war, a chemical attack on the outskirts of Damascus in August 2013 killed hundreds and nearly drove the United States to launch airstrikes against the government of embattled President Bashar al-Assad. The U.S. and Western allies accused Assad's government of being responsible for that attack, while Damascus blamed rebels.

A team of UN inspectors later determined that the nerve agent sarin was used in the attack.

Chlorine-filled cylinders

There have been several allegations that the Islamic State group has used chlorine as well. 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.

Elsewhere, Iraqi security forces engaged in fierce clashes with the militants as they continued their offensive to retake Saddam Hussein's hometown, Tikrit, on Saturday.

Iraqi forces, which include the military, police, Shia militias and Sunni tribesmen, entered the city of Tikrit for the first time 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 centre of Tikrit within two to three days.