Mass graves containing at least 400 suspected ISIL victims, many wearing prison uniforms, have been found in northern Iraq.

The sites were discovered on Saturday near Rashad airbase close to the group's former stronghold, Hawijah, 45km west of Kirkuk.

Iraqi forces recaptured the town of Hawijah and surrounding areas from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in October. It was one of their last strongholds in Iraq.

Rashad airbase, located about 30km south of Hawijah, was used by ISIL as a training camp and logistic base.

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Al Jazeera's Imran Khan, reporting from Baghdad, said it was a common ISIL tactic to imprison and execute anybody who was in opposition to their rule.

"The graves are a significant find, most of all for the families of the victims who have been seeking closure for years," Khan reported.

"According to locals, ISIL turned the military base into an execution ground. Iraqi authorities have been cataloguing exactly how many people died under ISIL rule and are now investigating how many victims are in these particular graves,” he added.

"Authorities are relying on local people who took detailed notes during ISIL rule, handed them over, and showed authorities where these graves are located."

The discovery of the mass graves was not the first after the routing of ISIL in Iraq and Syria.

In August, Iraqi military investigators discovered two mass graves near a former ISIL prison outside Mosul that contained the bodies of 500 victims.

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Earlier that month, Associated Press news agency conducted a survey that concluded that ISIL buried thousands of their victims in at least 72 mass graves across Iraq and Syria.

Through interviews, photos, and research, AP obtained locations of 17 mass graves in Syria, including one with the bodies of hundreds of members of a single tribe all but exterminated when ISIL took over their region.

Of the 72 mass graves documented in the investigation, the smallest contained three bodies; the largest is believed to hold thousands.