The Islamic State militant group has executed eight people, four of them policemen, in Iraq's Salaheddin province, according to a series of pictures posted online.

A message that accompanied the pictures, the authenticity of which could not be independently confirmed, said the eight men had reneged on a pledge to stop working for the Iraqi government.

It was not immediately clear when the photos were taken.

Seven of the victims can be seen wearing orange jumpsuits similar to those worn by journalists and aid workers killed by IS.

Captions identify them police officers, one is said to be a policeman and three are described as "secret informants".

The eighth victim is not identified.

Other pictures show the men blindfolded and marching towards what appears to be an area under a bridge across a river.

They are also seen lined up on their knees, with pistol-armed masked men wearing black uniforms standing behind them.

Another picture shows seven of the victims slumped forward with their hands bound, blood running from some of their heads, while the eighth is still upright, apparently as the militant behind him fires.

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

The brutal militant group has executed 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, Shiite militias, Sunni tribesmen and a US-led campaign of air strikes, have managed to regain some ground from IS.

But significant territory, including three major cities, are still controlled by the brutal militant group.