Access is severely restricted in large parts of Iraq's north and west, which ISIS militants have controlled since sweeping across the Syrian border in mid-2014 in a bid to establish a caliphate.
Witnesses and sources at a morgue in Mosul, the capital of Nineveh, told Reuters that most of the executions reported on Friday had occurred over the past six months.
The majority of the victims, killed for common crimes which had been buried earlier, but the bodies of journalists and former Iraqi police or soldiers were delivered to the morgue earlier on Friday, the sources said. They said ISIS had circulated lists containing the victims' names.
"(Islamic State) assassinated in cold blood ... 2,070 residents of Nineveh for ... not cooperating with them," Iraqi defence minister Khaled al-Obeidi said in a video posted on the ministry's website.
Interviews with residents in January revealed how ISIS had created a police state strong enough to weather severe popular discontent in Mosul, the country's second-largest city which fell to the insurgents.
Parliament Speaker Saleem al-Jabouri condemned the reported executions on Facebook as "an historical crime of genocide".