Iraqi government officials have said ISIL used chlorine gas during fighting with Iraqi soldiers and Shia militia north of Baghdad.

A senior security official, a local official from the town of Duluiya and an official from the town of Balad said the group used bombs with chlorine-filled cylinders during clashes in September.

They told the Associated Press on Friday that about 40 troops and militiamen were affected by the chlorine and showed symptoms of chlorine poisoning, but quickly recovered.

The Washington Post also reported on Friday that 11 police offices had been rushed to hospital about 80km north of Baghdad last month with signs of chlorine poisoning.

The US secretary of state, John Kerry, said on Friday that the US would investigate the claims, and said he took the allegations "very seriously".

ISIL has been accused of using chlorine gas in neighbouring Syria in attacks on other rebel groups and the forces of the Syrian president, Bashar al-Assad. Assad's army has also been accused of using the gas on civilians and rebels.

Chlorine is a "dual use" substance under international law - meaning it is an industrial substance that can be used on the battlefield. Syria kept its stocks of chlorine for this reason under last year's UN-brokered deal to destroy its chemical weapons.

Ongoing battles

Meanwhile, the Iraqi army has launched an offensive to retake towns held by ISIL south of Baghdad as it seeks to secure the Shia religious sites of Najaf and Karbala.

Local government sources told Al Jazeera that about 100 ISIL fighters surrendered on Friday around the town of Jarf al-Sakar. The army said it had already taken control of villages in the area.

Al Jazeera's Imran Khan said the operation and the events in the south were significant for several reasons.

"It is usual for ISIL fighters to fight to the death‎. We don't know if those towns were defended by core ISIL fighters or aligned Sunni groups," he said.

"But Jarf al-Sakar is the weakest link in the territory that ISIL control and it's the furthest south they've gone."

"The town and the surrounding area are important because of the proximity to the religious shrines in Najaf and Karbala.

"Those shrines are important to the Shias and if attacked represent a red line for the Iranians who have said they will send in troops to defend them."

The Iraqi army, backed by US air power, is also fighting north of Baghdad to retake the oil-rich area of Baiji and Tikrit.

The US central command said three air strikes south of the Baiji oil refinery struck two "small units" and destroyed an ISIL vehicle, and two other attacks struck a nearby ISIL training camp.