MOSUL, Iraq (Reuters) - When the boys first arrived at the Islamic State training facility in eastern Mosul they would cry and ask about their parents, who went missing when the militants rampaged through northern Iraq in 2014.

Math and English textbooks found in an Islamic State facility for child fighters, which include military imagery, are pictured in Mosul, Iraq, February 16, 2017. REUTERS/Khalid al Mousily

But as the weeks passed they appeared to absorb the group’s ultra-hardline ideology, according to a worker at the former orphanage where they were housed.

The children, aged from three to 16 and mostly Shi’ite Muslims or minority Yazidis, began referring to their own families as apostates after they were schooled in Sunni Islam by the militant fighters, he said.

The boys were separated from the girls and infants, undergoing indoctrination and training to become “cubs of the caliphate - a network of child informers and fighters used by the jihadists to support their military operations.

The complex in Mosul’s Zuhur district, which had been home to local orphans until they were kicked out by Islamic State, was one of several sites the jihadists used across the city.

It is now shuttered, its doors sealed with padlocks by Iraqi security forces.

Islamic State withdrew before Iraqi forces launched a U.S.-backed offensive in October to retake the city, but during a Reuters visit last month there were still reminders of the group’s attempt to brainwash dozens of children.

A saying attributed to the Prophet Mohammed is painted in black on one wall, urging children to learn to swim, shoot and ride horses. Inside the building is a swimming pool, now dry and full of rubbish.

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In another room sits a stack of textbooks Islamic State had amended to fit its brutal ethos.

Arithmetic problems in a fourth grade maths book use imagery of warfare, while the cover bears a rifle made up of equations. History books focus exclusively on the early years of Islam and emphasize martial events.

Another textbook entitled “English for the Islamic State” includes ordinary words like apple and ant beside army, bomb and sniper. Martyr, spy and mortar also appear alongside zebra crossing, yawn, and X-box.

The word “woman” is depicted by a formless black figure wearing the full niqab covering. All faces in the books - even those of animals - are blurred, in keeping with an Islamic proscription against such images.

The orphanage worker, who was cowed into staying on after the militants took over in 2014, said girls who were brought to the center were often married off to the group’s commanders.

The man asked not to be named for fear of reprisals by Islamic State, which still controls the entire western half of Mosul. He was shot in the leg during recent clashes.

He said the militants, mostly Iraqis, taught the Shi’ite children how to pray in the tradition of Sunni Islam and forced the Yazidis to convert.

They memorized the Koran, were taught to treat outsiders as infidels and conducted physical exercise in the yard, which has since grown over.

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A pair of colorful plastic slides and swing sets now sit untouched amid shattered glass, casings from a grenade launcher and a suicide bomber’s charred remains - signs of the militants’ fierce resistance as they retreated late last year.

Reuters could not independently verify the orphanage worker’s comments. But local residents gave similar accounts, and Islamic State has published numerous videos showing how it trains young fighters and even makes them execute prisoners.

New batches of children arrived at the Zuhur orphanage every few weeks from outside Mosul, including a few from neighboring Syria, while older boys were sent to the town of Tel Afar west of Mosul for intensive military training for duties including with Islamic State’s courts or vice squad, residents said.

“After six months at the camps, some of the boys came back to spend a weekend with their younger brothers. They were wearing uniforms and carrying weapons,” the orphanage worker said, fingering black and yellow prayer beads.

One of the boys, Mohammed, was killed last summer during the battle in the city of Falluja, west of Baghdad, he said, recounting how the other children wept upon learning the news.

A few weeks before the Mosul offensive began, Islamic State canceled lessons and sent the boys to guard an airfield near Tel Afar which pro-government forces later seized, he said.

“I told them, ‘If you see the army, drop your weapons and tell them you are orphans. Maybe they will spare your lives’”.