The Islamic State group burned to death nine of its militants accused of attempted desertion, dumping them into flaming oil trenches used to smoke out the advance of the Iraqi army-led offensive, local Iraqi media reported Tuesday.

The Iraqi Army, supported by Kurdish forces, Shiite militias and a U.S.-led coalition, has made significant gains in recent days toward Mosul, ISIS' second largest city and final stronghold in the country. The militants have responded by mounting fierce counterattacks. An ISIS sleeper cell assaulted the city of Kirkuk last week, forcing nearby Kurdish forces to redirect their attention from Mosul. The militants have also reportedly increased their use of suicide car bombings and grabbing women and children as human shields as Iraqi forces lead the charge deeper into ISIS territory.

“There is a grave danger that ISIL fighters will not only use such vulnerable people as human shields but may opt to kill them rather than see them liberated,” Zeid Ra’ad Al Hussein, United Nations High Commissioner for Human Rights, said in a statement last week.

ISIS has attempted to further complicate Iraqi-led military operations by setting fire to oil fields and a sulfur plant. The resulting toxic plumes have compelled U.S. coalition forces to provide around 24,000 gas masks to Iraqi and Kurdish forces participating in the offensive. The smoke provides ISIS fighters cover from Iraqi and coalition airstrikes, an attempt to disrupt a key advantage that the group's adversaries have enjoyed throughout the battle.

Reports of mass executions in Mosul and surrounding towns have increased since Iraqi Prime Minister Haider al-Abadi announced Oct. 17 the beginning of the latest offensive against the militants. As Iraqi forces and allies approach, many civilians and ISIS fighters have attempted to escape the city. They face certain death if caught. The group executed 284 males, adults and children, last week at the College of Agriculture located north of the city. ISIS also reportedly killed 18 of its own fighters in March for allegedly trying to surrender by text message to oncoming Kurdish forces in Kirkuk.