ISLAMIC State militants used chlorine gas during fighting with security forces and Shiite militiamen last month north of Baghdad, according to Iraqi officials.
The disclosure comes after similar reports from the Syrian border town of Kobani, indicating the extremist group may have added low-grade chemical ordnance to an arsenal that includes heavy weapons and tanks.
Yesterday, US Secretary of State John Kerry said: “These allegations are extremely serious and we are seeking additional information to be able to determine whether or not we can confirm it.
“The use of any chemical weapons is against international law and these recent allegations underscore the importance of the work we are currently engaged in.”
Mr Kerry insisted the possible gas attack would not influence US tactics toward IS. “Obviously, it can affect tactical decisions within that strategy, but our fundamental strategy remains absolutely clear,” he added.
The Washington Post reported that 11 Iraqi police officers were rushed to a hospital north of Baghdad last month with symptoms consistent with chlorine poisoning.
Iraqi forces have reported two other crude chlorine gas attacks since IS militants took over large areas of the country.
A local official from Dhuluiya and an official from Balad said the IS group used bombs with chlorine-filled cylinders in clashes while trying to take the towns in late September.
The officials said in the wake of the attacks that around 40 troops and Shiite militiamen showed symptoms consistent with chlorine poisoning, such as difficulty in breathing and coughing. The troops were treated in hospital and quickly recovered.
It was claimed the IS fighters obtained the chlorine from water purification plants in areas they have overrun.
Iraqi intelligence has indicated the group has shells filled with chlorine. “The IS fighters seized some quantities of chlorine after seizing control of water purification plants or sites where chlorine was kept,” said a senior Iraqi official, adding that the “IS group has some experts who were able to manufacture chlorine shells”.
A US-led coalition has bombed the Sunni jihadist group’s positions in Iraq since August, after IS fighters seized large swathes of territory in Syria and Iraq.
The US and other western nations have also accused president al-Assad’s forces of using chemical weapons in areas held by rebels in Syria’s civil war.
The Institute for the Study of War has compiled 18 allegations of chlorine gas attacks by the regime since US strikes against IS began in August.
The first strike was reported on 19 August – the same day a joint mission by the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW) said neutralisation of a chemical weapons stockpile surrendered by the regime had been completed.
The most recent attack is said to have taken place last week, when government forces allegedly used chlorine gas against rebels in the suburban Damascus area of Jobar.
In a deal worked out between the US and Russia last year, Damascus agreed to eliminate its chemical arsenal after a sarin attack in August 2013 killed hundreds in the outskirts of the capital.
The UN-OPCW mission had said all 1,300 tons of declared chemical weapons have been removed.
The disarmament operation had been hailed a success by US President Barack Obama, despite delays and Syria’s recent disclosure of four new secret chemical weapons facilities.
The US has raised concerns the weapons could fall into the hands of IS.