Islamic State (IS) crushed a rebellion by a former leader who planned to help Iraqi government forces and their allies regain control of the city, residents and Iraqi officials told Reuters.

The jihadist group executed 58 people involved in the plot. They were killed by drowning and their bodies buried in a mass grave in wasteland on the outskirts of the city. 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.

Among those killed was a close aide of IS chief Abu Bakr al-Baghdadi, who led the plotters, according to the sources.

Mosul has been IS's de facto capital in Iraq since the group seized swathes of the country in 2014. It is Iraq's second biggest city, with a pre-war population of about 2 million people.

It comes with Iraqi government forces and allied Kurdish militias poised to launch an operation to retake Mosul from the group. They have been backed with air strikes and military support from a US-led coalition.

The UN has warned that the operation could spark a humanitarian crisis, with one million people potentially uprooted. IS is expected to ferociously defend the city, and has previously used human shields and rigged areas with booby traps defending its territory.

The plotters – including several IS defectors – were reportedly stockpiling weapons to be used to support the Iraqi army in its mission to retake Mosul.

"Those were Daesh members who turned against the group in Mosul," Iraqi Counter-terrorism Service spokesman Sabah al-Numani in Baghdad told Reuters, using an Arabic acronym for Islamic State. "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."

Isis chief Abu Bakr al-Baghdadi is reportedly back in Mosul in northern Iraq
Islamic State chief Abu Bakr al-Baghdadi Reuters