The US has warned that Islamic State terror group can use mustard gas against an Iraqi offensive to retake Mosul, Iraq's second-largest city. US Defence Department said that the IS has already used mustard gas at least a couple dozen times and the terror group is likely to use it again. <br/><br/>Iraqi security forces, backed by US-led coalition air power, are in the final weeks of preparing operations ahead of an assault to recapture Mosul, which the IS seized in 2014. It remains the terror group's last main stronghold in Iraq.<br/><br/>The Pentagon suspects that the IS group used mustard gas last week in a rocket launched against US soldiers at a base near Qayyarah, a northern town in Iraq considered strategic for retaking Mosul. The base is in the process of being transformed into a logistics hub to support the Mosul offensive.<br/><br/>US officials estimate that Iraqi forces could be technically ready to launch the attack in early October.