ISIS fired a rocket filled with chlorine today in an attack in the west of the city which had largely been liberated from the extremists by US-backed Iraqi forces.
The news was confirmed by an Iraqi military officer who was speaking on the condition of anonymity as he was not authorised to release information.
The rocket is believed to have been launched in the al-Abar neighbourhood of the city.
Iraqi forces carry out a search in western Mosul
At least seven soldiers were reportedly suffering from breathing problems and were being treated nearby in a field hospital.
The Iraqi military is currently involved in an offensive in western Mosul, backed by US-led coalition forces.
Battles against jihadists are now ongoing in the more densely-populated western half of the city, with Iraqi officials claiming that more than half of western Mosul has been retaken from the terrorist group.
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The news comes after it was revealed earlier today that an ISIS militant who had been linked to the attack in 2015 on the French satirical magazine Charlie Hebdo could still be alive.
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Boubaker el-Hakim was reported by American defence officials to have been killed in November, in a US drone strike in Raqqa.
Iraqi intelligence supplied information to the Syrian airforce to carry out a series of strikes on ISIS headquarters and hideouts in Syria, including one believed to belong to el-Hakim, an Iraqi military statement said.
An Iraqi soldier searches the rubble in western Mosul
Aircraft from Syrian President Bashar al-Assad's airforce targeted several locations in Raqqa and Albu Kamal, near the Iraqi border, said the statement, without indicating the location of el-Hakim's headquarters or the date of the raids.
An Iraqi military spokesman told Reuters el-Hakim's headquarters were destroyed but it wasn't clear if he was killed.
In 2015 Iraq and Syria established a joint committee with Russia and Iran, Assad's main foreign backers, to share intelligence about Islamic State.
El-Hakim was believed to have been involved in planning the January 2015 attack on Charlie Hebdo, a weekly known for its satirical covers ridiculing political and religious leaders.
An Iraqi soldier in western Mosul
Two Islamist militants broke into an editorial meeting of the weekly, raking it with bullets, killing 17 people. Another militant later killed a policewoman and took hostages at a supermarket, killing four before police shot him dead.
ISIS declared a "caliphate" spanning parts of Iraq and Syria after it captured the Iraqi city of Mosul in mid-2014.
The hardline Sunni group has since lost most cities it captured in Iraq and its fighters are now surrounded in parts of Mosul by US-backed Iraqi government forces.
A US-backed offensive is also under way to capture Raqqa, involving a Syrian Kurdish-Arab militia alliance.