ΝΥΤ – BAGHDAD — Before war convulsed his hometown in Syria, Usaid Barho played soccer, loved Jackie Chan movies and adored the beautiful Lebanese pop singer Nancy Ajram. He dreamed of attending college and becoming a doctor.

His life, to say the least, took a detour.

On a recent evening in Baghdad, Usaid, who is 14, approached the gate of a Shiite mosque, unzipped his jacket to show a vest of explosives, and surrendered himself to the guards.

“They seduced us to join the caliphate,” he said several days later in an interview at a secret Iraqi intelligence site where he is being held.

Usaid described how he had been recruited by the Sunni extremists of the Islamic State from a mosque in his hometown, Manbij, near Aleppo. He said he joined the group willingly because “I believed in Islam.”

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In the same video in which Usaid re-enacted his surrender, the mosque where he gave up is shown.

He soon found himself in Iraq, but he quickly had misgivings and wanted to escape. His best chance, he decided, was a risky deception: volunteer to be a suicide bomber so he could surrender to security forces.

The wars in Syria and Iraq have set grim new standards for the exploitation and abuse of children. Thousands of them have been killed or maimed through indiscriminate bombings, in crossfire and, in some cases, executions. Young girls from minority groups, especially Yazidis in Iraq, have been captured as sex slaves by the Islamic State, also known as ISIS or ISIL. Young boys have been given rifles and told to staff checkpoints or patrol neighborhoods — or have been recruited, as Usaid was, to become suicide bombers.

In the areas it controls in Iraq and Syria, the Islamic State has established centers for the military and religious training of children, in an effort to indoctrinate them and build a new generation of warriors.

One of the group’s videos, depicting a camp near Mosul, in northern Iraq, calls the children the “cubs of the caliphate.” At the camp — named for the brutal leader of Al Qaeda in Iraq, Abu Musab al-Zarqawi, who was killed by an American airstrike in 2006 — children are shown doing physical fitness exercises and reciting the Quran, while an instructor explains that they are being trained to fight “hate-filled Shiites.”

The United Nations wrote in a report last month that “ISIS prioritizes children as a vehicle for ensuring long-term loyalty, adherence to their ideology and a cadre of devoted fighters that will see violence as a way of life.”

The United Nations has released a catalog of horrors inflicted on children by the Islamic State. In Raqqa, Syria, the militants’ de facto capital, the group has gathered children for screenings of execution videos. It has forced children to participate in public stonings. And in many of the group’s grisly execution videos, children are seen among the audience. (Usaid said that his parents did not allow him to attend the public executions in his town, typically held after Friday Prayer.)

In the aftermath of one videotaped beheading in Deir al-Zour, Syria, children are seen playing with the victim’s head and mocking the corpse, according to the SITE Intelligence Group, which monitors the communications of extremist groups.

Referring to past wars and the role of children, Laurent Chapuis, the regional child protection adviser for the Middle East and North Africa for the United Nations Children’s Fund, or Unicef, said: “When it comes to recruitment, in the past, kids were predominantly supporters — messengers or spies. It seems now they are pushed to take a more active role.”

Mr. Chapuis said that all parties in the wars, including pro-government militias in both countries, were guilty of abuses of children. What sets the Islamic State apart, he said, is how “public and aggressive” they are in their exploitation.

Usaid’s account of how he went from a Syrian childhood that he said was not particularly religious to become a jihadist held in an Iraqi cell is one of the few firsthand accounts from an Islamic State child soldier turned defector.

The Iraqi authorities have increasingly showcased Islamic State detainees to the public, as part of a strategy to demonstrate  that the government is making progress in the fight, although they have not typically made detainees available for interviews with journalists. The details of Usaid’s personal background could not be independently verified, but his surrender at the mosque gate was captured on cellphone video by a bystander.

First, after the Islamic State took control of his town, Usaid was drawn to the local mosque. “We started being taught that Shiites were raping Sunni women, and that Shiites were killing Sunni men,” he said.

He now says he was brainwashed. But he admitted that he willingly ran away from home one morning on his way to school and joined a training camp in the desert. For about a month, he was put through military training, and he was taught how to shoot an assault rifle and how to storm a building. He had two meals a day, mostly cheese and eggs.

Soon, though, he said, “I noticed things I saw that were different from Islam.”

Back home he saw the group inflict severe punishments on men who were caught smoking cigarettes, yet in the camp, he said, he saw fighters smoking. He said he saw men having sex with other men behind the tents in the desert night. And, he said, he was increasingly put off by “the way they are killing innocent people.”

At the end of the training, he was told his trainers wanted him to go fight in Iraq. He was driven, with other new fighters, in a minibus to Mosul.

There, the recruits were given a choice: be a fighter or a suicide bomber.