The officer with the anti-terrorism forces said today that the attack occurred the night before in the al-Abar neighbourhood, when IS fired a rocked loaded with chlorine.
He said seven soldiers suffered breathing problems and were treated in a nearby field clinic.
The officer spoke on condition of anonymity as he was not authorised to release information.
US-backed Iraqi forces are currently battling IS militants in the more densely-populated western half of Mosul.
Iraqi officials say more than half of western Mosul has been retaken.