BAGHDAD (AFP) - The Islamic State in Iraq and Syria (ISIS) group has publicly beheaded three men in northern Iraq, two of them for allegedly engaging in homosexual acts, according to photos shared by sympathisers on social media Tuesday.

The extremist group, which controls swathes of Iraq and neighbouring Syria, has carried out hundreds of executions as it has imposed its brutal version of Islamic law, many of them photographed or videotaped.

The latest images did not show the claimed beheadings and their authenticity could not be independently verified.

A series of photographs shows the blindfolded men kneeling in the centre of what appears to be a traffic circle with a crowd of people looking on as a masked, black-clad executioner stands by with a long, rusty blade.

Accompanying captions said the trio were then executed, the third of them for alleged blasphemy.

The photos were said to have been taken in Nineveh province - the first to fall to a sweeping ISIS-led offensive last June - but the exact location was not specified.

Iraqi troops and militia are battling to push the militants back with support from a US-led coalition and Iran.

But retaking Nineveh and its capital Mosul poses a major challenge for Baghdad's forces, as the militants have had more than nine months to dig in.

Iraqi forces launched a huge operation last week aimed at retaking the city of Tikrit that, if successful, would serve as a stepping stone towards Mosul.