Daesh killed 58 people earlier this month and crushed a rebellion plot in Mosul, led by one of the group's commanders, residents and Iraqi security officials said.
Those executed included several members of the terrorist organisation who were suspected of taking part in the plot, that was uncovered last week.
The plotters, who aimed to switch sides and weaken Daesh's defences to help deliver Mosul to the Iraqi government, were killed by drowning and their bodies were buried in a mass grave, according to several residents of the area.
The head of the plotters was a local former assistant of Daesh leader Abu Bakr al-Baghdadi working together with ex-Daesh members who turned against the group.
The details were obtained from matching accounts given by five residents, by Hisham al-Hashimi, an expert on Daesh affairs who advises the government in Baghdad and by Colonel Ahmed al-Taie, from Mosul's Nineveh province Operation Command's military intelligence.
The rebels were discovered after one of them was caught with a message on his phone mentioning a transfer of weapons, Hashimi said.
After interrogation he gave out three locations of hidden weapons, that were to be used in the rebellion to support the Iraqi government forces when they closed in on Mosul.
Daesh raided the three houses on October 4, Hashimi said.
"Those were Daesh members who turned against the group in Mosul," said Iraqi Counter-terrorism Service spokesman Sabah al-Numani in Baghdad.
"This is a clear sign that Daesh has started to lose support not only from the population, but even from its own members."
A spokesman for the US-led military coalition which conducts air strikes on Daesh targets in Syria and Iraq was unable to confirm or deny the accounts of the thwarted plot.
Some people in Mosul have been expressing their refusal of Daesh’s harsh rules by spray-painting the letter M, for the Arabic word "Mukawama" that means resistance, on city walls, or "wanted" on houses of its militants.
Daesh punished such activity by death.
Numani said his service has succeeded in the past two months in opening contact channels with "operatives" who began communicating intelligence that helped conduct air strikes on the insurgents' command centres and locations in Mosul.
A list with the names of the 58 executed plotters was given to a hospital to inform their families but their bodies were not returned, the residents said.
"Some of the executed relatives sent old women to ask about the bodies. Daesh rebuked them and told them no bodies, no graves, those traitors are apostates and it is forbidden to bury them in Muslim cemeteries," said one resident whose relative was among those executed.
A Mosul resident said Daesh had appointed a new official, Muhsin Abdul Kareem Oghlu, a leader of a sniper unit with a reputation as a die-hard, to assist its governor of Mosul, Ahmed Khalaf Agab al-Jabouri, in keeping control.
Mosul is the last major stronghold of Daesh in Iraq. With a pre-war population of around 2 million, it is at least five times the size of any other city Daesh has controlled.
Daesh militants have placed booby traps across the city of Mosul, dug tunnels and recruited children as spies in anticipation of the offensive.
Iraqi officials say a massive ground assault could begin this month, backed by US air power, Kurdish security forces and Shia and Sunni irregular units.
A successful offensive would effectively end control of the group in Iraq. But the United Nations says it could also create the biggest humanitarian crisis in the world, in a worst case scenario uprooting 1 million people.