Updated

Iraqi officials are accusing Islamic State militants of using chlorine gas against security forces and Shiite militia during last month’s clashes north of Baghdad.

If confirmed, this would be the first time Sunni extremists attempted to use chlorine since their blitz that seized large chunks of northern Iraq and Syria territory, earlier this year.

U.S. Secretary of State John Kerry said that he couldn’t confirm use of chlorine gas in the attack and calls such weapons “abhorrent and against law.”

But three Iraqi officials —a senior security official, a local official from the town of Duluiya and an official from the town of Balad — told the AP that  Islamic State militants used chlorine-filled bombs during clashes in late September.

The militants have failed to capture both Duluiya, 45 miles north of Baghdad, and Balad, 50 miles north of the Iraqi capital.

In the attacks, about 40 troops and Shiite militiamen were slightly affected by the chlorine and showed symptoms consistent with chlorine poisoning, such as difficulty in breathing and coughing, the three officials said. The troops were treated in hospital and quickly recovered.

The senior security official said it was most likely that the Islamic State fighters obtained the chlorine from water purification plants located in the areas they had overrun earlier. Iraqi intelligence has indicated that the IS group has some shells filled with chlorine and they are ready to be used, the official also said.

"The IS fighters seized some quantities of chlorine after seizing control of some water purification plants or sites where chlorine was kept," said the senior official, adding that the "IS group has some experts who were able to manufacture chlorine shells."

The three officials, who spoke on condition of anonymity because they were not authorized to speak to media, did not elaborate or provide more details. The use of chlorine by the IS group in Iraq in September was first reported by the Washington Post.

Chlorine, a chemical used in industry and water purification process, was first introduced as a chemical weapon at Ypres in World War I with disastrous effects because gas masks were not easily available at the time.

In neighboring Syria, a joint U.N. fact-finding mission sent to investigate alleged chlorine attacks was ambushed and briefly detained by armed men earlier this year in rebel-held territory. The mission had said it was virtually certain chlorine had been used as a chemical weapon in the country's north.

Syrian President Bashar al-Assad had agreed with the United States and Russia to dispose of his chemical weapons — an arsenal that Damascus had never previously formally acknowledged — after hundreds of people were killed in a sarin gas attack on the outskirts of the capital, Damascus, in the summer of 2013. But chlorine was not listed as part of the Syrian arsenal.

Chlorine gas, when inhaled, swallowed or exposed to through skin causes difficulty in breathing, coughing, and eye and skin irritation. It is not as toxic or effective at killing as sarin, a nerve agent, and experts say it is difficult to achieve high concentrations of chlorine by dropping it from the air.

In 2011 in Baghdad, hundreds of people were sickened by a chlorine gas cloud that leaked over the Iraqi capital after an explosion at a water treatment plant.

The Associated Press contributed to this report.