BAGHDAD (AFP) - The Islamic State in Iraq and Syria (ISIS) group strung up four Iraqi Shiite fighters with chains and burned them alive, according to footage posted online, the latest gruesome execution video from the militants.

The victims - identified as fighters in the pro-government Popular Mobilisation forces from southern Iraq - were suspended from a swingset-like metal structure by chains attached to their hands and feet, then set on fire.

ISIS, which overran large parts of Iraq last year and still controls much of the country's west, said the murders were in revenge for the alleged burning of four men by pro-government forces.

"Now retribution has come, for today, we will attack them as they attacked us, and punish them as they punished us," a masked militant says in the video.

The video was not dated and did not give a specific location for where the killings took place, but it did carry a tag indicating that it was produced by the ISIS media unit responsible for Iraq's Anbar province.

The video included a clip said to show a Sunni man suspended over a fire while still alive as pro-government forces look on, and another of famous Shiite fighter Abu Azrael ("Father of the Angel of Death") slicing a piece of flesh off a burned corpse with a sword.

ISIS has carried out a slew of atrocities in territory it controls in Iraq and Syria, such as mass executions and a campaign of killings, kidnapping and rape targeting minorities.

It has recorded many killings - including beheadings, shootings, drownings and burnings - in videos posted online.

Baghdad's forces regained significant ground from the jihadists in two provinces north of the capital with support from a US-led coalition and Iran, but much of western Iraq remains outside government control.