The Islamic State group has crushed a rebellion plot in its stronghold city of Mosul in Iraq, Reuters reported Friday.

The conspiracy was reportedly lead by one of Islamic State leader Abu Bakr al-Baghdadi's own aides, who along with 57 others, were executed for treason. The accused were drowned and their bodies dumped in a mass grave located somewhere in the barren outskirts of the city. The militants also known as ISIS responded to inquiries by family members regarding the whereabouts of their loved ones by refusing to provide any details except for a list of names of the "apostates" and indicating that they could not be buried in Muslim cemeteries.

The operation was revealed by a text message discovered on one of the rebels' phones discussing weapons transfers, according to Iraq government advisor Hisham al-Hishami and Iraqi colonel Ahmed Al-Taie. The saboteur was then interrogated until he confessed knowledge of three weapon caches hidden throughout the city. The dissidents planned to use the arms to attack the self-proclaimed caliphate's largest city from within, undermining ISIS' defense as the Iraqi army, Kurdish forces, the United States and Iranian-supported Shiite militias prepare to assault the city in what may be the nation's largest battle since the U.S. invasion in 2003.

“Those were Daesh members who turned against the group in Mosul," Iraqi Counterterrorism Service spokesman Sabah al-Numani in Baghdad told Reuters, using an Arabic acronym for ISIS. "This is a clear sign that the terrorist organization has started to lose support not only from the population, but even from its own members.”

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Residents of Mosul had previously expressed their dissatisfaction with ISIS rule by spray painting the Arabic letter "meem" (M) for "moqawema" meaning "resistance." The Islamic State group took the city, which had a pre-war population of about 2 million, in 2014 after a number strategic victories sent the Iraqi army fleeing. The victory sent shock waves throughout Iraq as the world watched a terror group transform into a proto-state covering parts of Iraq and Syria. Since then, however, ISIS has lost a number of key cities including Ramadi in Iraq and Deir al-Zour in Syria after Iraqi and Syrian army offenses this year.

The operation against the militants in Iraq has sparked unusual coordination between the Iraqi army, Kurds, Turkey, the United States and Iran. Iraq has previously threatened Turkey not to get involved in the conflict and Iranian-U.S. efforts in Syria are entirely at odds.