ISTANBUL — Mohammed, 14, uses a wheelchair to move around the tiny apartment where he lives in the Turkish city of Sanliurfa with his brother and a friend of the family who acts as his nurse.

The apartment is on the third floor of a dilapidated walk-up building. It has no air conditioning. The doorways in the apartment are narrow. His wheelchair doesn’t fit through all of them, so Mohammed, originally from eastern Syria, often pushes himself out of the chair and hops around the apartment on one foot.

ISIS chopped off Mohammed’s right hand and left foot two weeks ago. ISIS tried to turn Mohammed into a child soldier. The group disfigured him because he refused to cooperate.

Ahmed is another boy from the same part of Syria as Mohammed, but is two and a half years older than him. Ahmed says ISIS coerced him into fighting for the group with a combination of threats and false promises.

Interviews by NBC News with Mohammed, Ahmed, their guardians, Syrian activists and a former ISIS commander who defected, reveal a disturbing ISIS strategy to recruit and deploy an army of child soldiers, brainwashing them until they are ready to blow themselves up or carry out terrorist attacks in the Middle East and abroad, and brutalizing those who refuse to cooperate.

Much has been written about ISIS’s monstrous treatment of girls, forcing them to become child brides or sex slaves, passed from fighter to fighter. This is a story about what ISIS is doing to the boys in areas under its control.

Rebel boys

Although Ahmed and Mohammed both look like typical young teenagers, pimples and all, the boys were both part of rebel groups fighting against the Syrian regime and ISIS. Mohammed worked as a spotter, using binoculars to help the rebels locate their targets.

“First we were going to (anti-government) demonstrations. Later on we got armed with the Free (Syrian) Army, and we fought the (Bashar) al-Assad regime for three years,” Mohammed says with obvious pride in his voice.

Ahmed started out as a helper, serving the rebels meals and running errands, until he eventually earned enough respect to be given a gun.

The balance of power shifted in in eastern Syria in 2014. After ISIS took over the Iraqi city of Mosul, the group had renewed enthusiasm and new American-made weapons abandoned by the Iraqi army when it broke ranks and collapsed in cowardice. ISIS quickly took over eastern Syria and began its reign of terror. Mohammed, Ahmed and other boys who worked with the rebels went into hiding.

“I stayed at home for seven months,” Mohammed said. “Later on, ISIS started arresting members of the Free (Syrian) Army. One of the detainees told them that I was also member.”

Mohammed didn’t know he’d been informed on until a group of ISIS fighters showed up at his door to seize him.

An image taken from ISIS video purportedly shows Al Farouk training camp for "cubs" [children]. The camp is in Raqqa, Syria, according to Flashpoint Intelligence, a global security firm and NBC News consultant. ISIS video

Ahmed managed to escape eastern Syria when ISIS moved in, but found himself in an unfamiliar part of the country without friends, money or options.

“ISIS guaranteed if we returned, they would guarantee our safety,” Ahmed said. “I needed to see my family. (ISIS) said if I go back, give up my weapon and that I wouldn’t be harmed.” He took ISIS at its word and returned to eastern Syria.

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After Mohammed was discovered and arrested, he was locked in an ISIS jail for two months. He says he was in a room with 75 other boys and men. He says they were all tortured savagely, beating his shins with bats and applying electric shocks to his genitals.

“Many people died there. No water, no electricity. They provided water twice a day. We used the toilet once a day,” Mohammed said.

Ahmed found out ISIS’s offer of amnesty came with conditions. After returning to eastern Syria, he was ordered to “repent” by going an ISIS indoctrination school. He says special attention was given to boys like him for brainwashing.

“They talked about Osama bin Laden, and how he attacked the twin towers. They were saying that they need to do that again,” Ahmed said. “They also talked about the attack on the French newspaper (Charlie Hebdo). They said this was made by the Islamic State. They were so happy.”

After behind tortured, Mohammed was released from prison and also sentenced to repent by going to the ISIS indoctrination school. But Mohammed was convinced that if he went to the school, he’d be sent to the front lines to die or used as a suicide bomber. Mohammed decided to try to escape with a group of friends.

One of them turned him in.

'This is the judgment of God'

Within a few hours, Mohammed was brought before an ISIS judge. His crime was trying to escape ISIS-held areas, thereby choosing to leave the realm of “true Islam” for the land of the infidels, which ISIS considers everywhere else.

“He said to me ‘This is the judgment of God. You were going to the land of the infidels… so you are like them. Your leg and arm must be cut off.’”

The punishment was carried out the next day. What Mohammed described is graphic and may be difficult to read, but it demonstrates ISIS’s systemic brutality against those who oppose it, including young boys.

The "chopping" ritual is carried out in a carnival-like atmosphere with an assembled crowd cheering and jeering as the condemned are brought to a city square. A section of honor in the square is reserved for the children of foreign fighters.

The sons of foreign fighters are expected to cheer and jeer the loudest. They’re being trained, both Ahmed and Mohammed say, to be elite members of ISIS’s next generation of suicide bombers and foreign terrorists. Sometimes the sons of foreign fighters carry out executions themselves, erasing their respect for human life while they’re still impressionable.

Mohammed’s punishment was to have his opposing limbs cut off: his right hand (considered in Islamic culture to the “clean” hand) and his left foot. ISIS attempts to make the horrific ritual appear hygienic. The ISIS members who held Mohammed’s limbs in place wore surgical gloves and squirted iodine on his hand and foot. Mohammed says he was given an injection. He doesn’t know what it was but says he was told it would calm him down.

The "chopping" ritual is carried out in a carnival-like atmosphere with an assembled crowd cheering and jeering as the condemned are brought to a city square.

ISIS members tied Mohammed’s arm and leg with tourniquets to cut off the blood flow to his hand and foot. The purpose of the amputations is not to kill the victim, but maim him forever. The tourniquets were left on for about 15 minutes. Then, Mohammed’s arm was stretched out on a block of wood. A large meat cleaver was placed on top of his wrist at the point where the cut would be made. A man with a mallet smashed the back of the cleaver so it would cut straight through bone and flesh. Mohammed and three others identified the man who carried out the punishment as an Iraqi from ISIS’s “chopping committee” known by the nickname “The Bulldozer.”

The first chop was met with celebratory cries of Allahu Akbar — God is greatest.’ The ritual was repeated to cut off his foot. Mohammed was then taken to an ISIS clinic where his skin was stretched and sewn over the stumps. Mohammed’s mother picked him up from the clinic, brought him home and smuggled him to Turkey a few days later.

'Who wants to be a martyr?'

After indoctrination school, Ahmed thought he’d finally paid his dues and that ISIS would make good on its promise of amnesty. Unlike Mohammed, Ahmed was cooperating. But ISIS kept adding demands. As the brainwashing course was wrapping up, Ahmed was told he was now an ISIS fighter and was being sent to the frontline. With no idea where he was going, Ahmed was loaded on a bus and taken to Anbar province in Iraq. He managed to escape from there to Turkey.

Ahmed says ISIS repeatedly tried to convince him and the other boys at indoctrination school to become suicide bombers. Some agreed.

“They asked us, “Who wants to be a martyr?’ One of the boys who was with us, he was 12 years old. He blew himself up in an area called Haditha (in Iraq),” Ahmed said.

Mohammed and Ahmed both say ISIS is trying to create a fresh crop of radicals by targeting boys.

“They are fooling children with money,” Mohammed said. “They are fooling them to bomb themselves. They give a boy some money, or a bicycle, and after two days they take him in a car to bomb himself. They target children the most. They focus on children, because children are unaware of anything in this life.”

Ahmed today is trying to make his way in Turkey, one of around two million Syrian refugees in the country.

Mohammed doesn’t have enough money for the medical and psychological care he badly needs. His brother and the family friend who nurses him do the best they can with the help of a few generous Turks. Mohammed was disfigured by ISIS only two weeks ago. He cries every time his bandages are changed. He has nightmares, wets his bed and gets confused. Last week, he forgot his foot was missing and fell down the stairs of the building where he is living. His brother takes him outside every day to get some fresh air.

"They target children the most. They focus on children, because children are unaware of anything in this life.”

“I can only sleep after I take sleeping pills,” Mohammed said. “The most difficult time is when I go to sleep. The pain starts. In the daytime, I sit in the street and watch the people passing by to forget the pain.”

But Mohammed says what hurt more than falling down the stairs was when a Turkish man approached him and gave him 50 Turkish lira, the equivalent of about 15 dollars. Mohammed wasn’t begging. He was just sitting his wheelchair, as he says, to forget the pain.

When he looked at the money in his lap, he started to cry. It dawned on him that he’d become what in Arabic is called a “miskeen,” someone who deserves pity. That’s not how Mohammed sees himself, but he realized that’s how others see him.