A Syrian family who escaped ISIS' de facto capital of Raqqa has described seeing an 11-year-old boy's arm chopped off as punishment by the terror group's religious police.

The family, who chose to remain anonymous for fear of retribution against them or their family members, told Sky News the boy was a orphaned neighbor who attempted to exchange a stolen car battery for food. The man he tried to bargain with informed the ISIS authorities.

''They used a sword … they put a block of wood underneath his hand, put his hand on it and said this stole a car battery … they didn't say he was an orphan," the family's mother said. "They said this hand has stolen and he had to be punished and they cut his hand off."

Another mother spoke of her anguish at being pressured into "selling" her 14-year-old daughter in marriage to an Egyptian ISIS fighter who was 29.

The woman was trying to bring up her four sons and daughter alone after her husband was killed in an airstrike. She said the ISIS fighter offered her $500 worth of gold in return for her daughter.

"My daughter said, 'My mother, do you think if my father was alive, he would accept this marriage?'" the woman said.

The teenager told Sky News, "He told me to carry a weapon and to stay with them. I told him, 'please I want to leave now."

"She is a baby. How can they let her carry a weapon?" her mother added. "At her age, she should be in the ninth grade. She is still playing on the swings with her little friends. She's not interested in guns. When you enter their areas, it is terrifying: you see ammunition and weapons."

There are growing signs of tension among the fundamentalists in the city - and an increase in people attempting to flee.

There are reports too that ISIS fighters are building tunnels in expectation of a tremendous battle to try to retake the Syrian city.

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