The killings of the men, who wanted to join anti-IS paramilitary forces known as Popular Mobilisation units, took place in the town of Hawijah, a police intelligence officer and two local officials said.
The executions could not be independently confirmed, but a gruesome series of photos posted online and shared on social media are evidence that they took place.
The photos show the bodies of more than a dozen different men strung by their feet from light poles, what appears to be a communications or electricity tower, and under a massive sign featuring the IS flag and name.
IS spearheaded a sweeping offensive last June that overran large parts of the country north and west of Baghdad, including in Kirkuk province.
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Security forces backed by militia are battling to regain ground, with support from a US-led coalition and Iran.
The Popular Mobilisation units -- which are dominated by Shiite militia groups but also include other volunteers, including Sunnis -- have played a key role in the anti-IS fight, shoring up the government's flagging troops.
But Shia militiamen have also been accused of carrying out atrocities targeting Sunni Arabs in areas retaken from IS.