For insulting Islamic State (ISIS) supremo Abu Bakr al-Baghdadi by calling him a "dog," an Iraqi man was reportedly beheaded by his own son, who is an ISIS militant.

The execution took place in the ISIS stronghold of Mosul, which is now under siege from Iraqi forces, aided by U.S.-led coalition forces.

In a report that first came out from the Arabic language Iraqi news outlet Al-Sumaria, the unidentified ISIS militant was said to have killed his own father also because the latter criticised the group's violent Muslim ideology, according to BAS News.

The report said the beheading that occurred last week was watched by a large crowd in Mosul's city centre, The Christian Post reported.

Repugnant as it was, this particular kind of execution is not something new as far as ISIS is concerned.

In August, another ISIS militant beheaded his own father reportedly because the latter allowed his family to flee Mosul, which has been under ISIS control since the summer of 2014, according to Iraqi News.

In that particular incident, the militant was so consumed by anger that he reportedly even filmed his execution of his own father.

In January, a 20-year-old ISIS militant, this time in the group's Syrian stronghold of Raqqa, executed his own mother whom he accused of "apostasy." The British-based Syrian Observatory for Human Rights said the mother was executed in front of the post office where she worked, watched by hundreds of residents.

The watchdog group reported that the mother was handed the penalty of death for simply "inciting her son to leave the Islamic State."

Meanwhile on the battlefront, six ISIS patrols were attacked by Mosul residents within a five-hour period on Tuesday, Iraqi News said, translating the Arabic language report from Al Sumaria.

Also on Tuesday, a group of young rebels attacked an ISIS headquarters in Mosul, killing two guards and burning a vehicle loaded with heavy weapons.

The youngsters chanted "Mosul is free" and raised the Iraqi flag before withdrawing from the area as ISIS fighters arrived to regain control of their headquarters.