Islamic State group fighters killed nine children by dismembering them with chainsaws in front of a crowded city square, British tabloids reported Thursday. The youths were punished for belonging to a group that opposes the militants also known as ISIS.

The children were reportedly chained to an iron pole in Tal Afar Square in Mosul, ISIS' stronghold in Iraq, before they were slaughtered. The youths' gender and ages were not known.

“The death sentence pronounced by ISIS sharia court stated that the men should be tied to an iron pole in the center of Tal Afar Square in Mosul and then sliced into two with an electric chainsaw,” an unnamed source told the Daily Star.

ISIS is known for its brutal killings. Its members have beheaded enemies for refusing to covert from Christianity, smoking cigarettes, having gay relations and listening to Western music, among other so-called crimes. Mosul residents who do not cooperate with the militant group are routinely killed in public executions.

Airstrikes carried out against ISIS in Syria and Iraq by Russia and U.S.-allies have reportedly only further enraged the militant group. As ISIS loses territory in both Syria and Iraq, it has become more violent against residents in the cities and towns it occupies.

In August, ISIS fighters detained 3,000 people fleeing their homes in northern Iraq and reportedly killed 12 of them, according to the United Nations. ISIS used other captives as human shields while fighting the Iraqi military.

"Al Qaeda wants Muslims to believe that its terrorism is morally justifiable, whereas the Islamic State argues that only its followers have moral legitimacy," Thomas Joscelyn, a senior fellow at the Foundation for Defense of Democracies who studies jihadist groups, told the New York Times.

UN Secretary-General Ban Ki-moon has said that ISIS is guilty of "heinous crimes" that may constitute war crimes and genocide.