BAGHDAD (AFP) - The Islamic State in Iraq and Syria (ISIS) beheaded an Iraqi police officer and a soldier, the latest in a long series of atrocities committed by the extremists, according to pictures posted online Sunday.

In one photo, a blindfolded man said to be a police lieutenant colonel kneels in a street in front of a row of gunmen.

A masked militant is then shown beheading the victim with a machete, after which his severed head is placed atop his body and the machete driven into his back.

Another photo shows an overweight masked militant apparently struggling to behead a man identified as a captured Iraqi soldier.

The caption says a second soldier was also beheaded, but his death is not shown.

The authenticity of the photos could not be independently confirmed.

ISIS spearheaded a sweeping militant offensive in June that overran large areas north and west of Baghdad, and also holds significant territory in neighbouring Syria.

The brutal militant group has killed thousands of people in both countries, some of them in highly-choreographed videotaped sequences in which the victims are beheaded.

Iraqi security forces, backed by Kurdish troops, Shi'ite militias, Sunni tribesmen and a US-led campaign of air strikes, have managed to regain some ground from ISIS.

But the militants still control significant territory, including three major cities.