GENEVA – Members of the Islamic State have carried out war crimes on “a massive scale” against captured fighters and civilians in areas of northeast Syria under its control, United Nations investigators said Friday.

“ISIS has beheaded, shot and stoned men, women and children in public places in towns and villages across northeast Syria,” according to a report by the U.N.’s International Commission of Inquiry on Violations in Syria. It said the conclusions were based on more than 300 interviews with victims and eyewitnesses who fled and on photographs and videos.

Paulo Pinheiro, the Brazilian chair of the commission, called the testimonies shocking and said they formed important evidence that the Islamic State has committed war crimes and crimes against humanity to silence and subjugate Syrians living under its control.

The group uses the practice of “takfir,” declaring someone to be a heretic, in order to justify attacks on any individual or group it perceives to be a challenge to its power, the report said.

According to eyewitness accounts, people caught eating during the fast of Ramadan were lashed in the streets. In one incident recounted in the report, “an ISIS member approached a 14-year-old boy after seeing him drinking water, then dragged him to the middle of the crowd in the street, announced his crime and lashed him 79 times.”

The Islamic State, through its al Hishah morality police, informs residents in advance of planned executions and “urges them to attend,” the report said. “Following the killings, the corpses are placed on public display, often on crosses for up to three days, serving as a warning to local residents.”

Witnesses reported scenes of heads placed on spikes along park railings, the report said.

Fraternizing with members of the opposite sex can be fatal, the report said. The Islamic State “has executed women as well as men for unapproved contact with the opposite sex, resulting in charges of adultery.” The report documents the execution of eight women for adultery in June and July 2014.

In August, the report said, the Islamic State beheaded a female dentist in the town of al Mayadin “because she had continued to treat patients of both sexes.”

The Islamic State also has prevented the delivery of humanitarian supplies and medicine in Deir el Zour, Raqqah and Hasakah provinces.

The group also pays special attention to “indoctrinating” children “to entrench its militant ideology” in an effort to use children “as a vehicle for ensuring long-term loyalty … and a cadre of devoted fighters that will see violence as a way of life.”

“In Raqqah city, children are gathered for screenings of videos depicting mass executions of government soldiers, desensitizing them from extreme violence,” the report said.

The group attempted to indoctrinate 153 Kurdish boys it had abducted in May, detaining them in a school in Minbij in Aleppo province and subjecting them to daily instruction in militant ideology for five months, before releasing them back to their families. Parents of the boys described fearing their sons “were deliberately groomed to inject ISIS’s worldview into their Kurdish communities,” the report said, using an acronym for the Islamic State.

The report also provides chilling accounts of executions and mutilation of large numbers of captured Syrian soldiers, including hundreds captured after the fall of the Taqba airbase in late August.

“Over two hundred men were stripped to their underwear and forced to walk into the desert,” the report said. A video later showed hundreds lying dead in the sand, with gunshot wounds to the head.