The victims - identified as fighters in the pro-government Popular Mobilization forces from southern Iraq - were suspended from a swing set by chains attached to their hands and feet, then set on fire.
According to media reports, these fighters were burnt alive 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, which was not dated and did not say where the burnings took place.
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.
OneIndia News (with inputs from agency