KIRKUK, Iraq (AFP) - The Islamic State in Iraq and Syria (ISIS) executed 20 people who wanted to fight against them in the northern Iraqi province of Kirkuk, officials said on Monday.

The killings of the men, who wanted to join anti-ISIS 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 ISIS flag and name.

Captions under the photos said the men were members of the Popular Mobilisation units and used a derogatory term that could either refer to the units' Shi'ite leadership, or indicate that the men were believed to be Shi'ites.

ISIS spearheaded a sweeping offensive last June that overran large parts of the country north and west of Baghdad, including in Kirkuk province.

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 Shi'ite militia groups but also include other volunteers, including Sunnis - have played a key role in the anti-ISIS fight, shoring up the government's flagging troops.

But Shiite militiamen have also been accused of carrying out atrocities targeting Sunni Arabs in areas retaken from ISIS.