The baby of Shamima Begum, a British teenager who joined the Islamic State and has sought to return home even as she has defended the group’s actions, has died in a detention camp in Syria, according to a spokesman for the group currently holding the woman.

Now 19, Ms. Begum was a 15-year-old schoolgirl when she slipped out of her parents’ home in the Bethnal Green district of London and joined two of her friends as they headed to Syria to pledge allegiance to the Islamic State, or ISIS. She had lived inside the group’s territory for four years, married a fighter and had two children, both of whom she said died of malnutrition and were buried in ISIS’s last remaining village in Syria, which has now been encircled by coalition forces.

She was pregnant with her third child when she fled the group’s territory and surrendered to an American-backed group, the Syrian Democratic Forces, which brought her to a detention camp. She was held there alongside other fighters’ wives, and was found by Anthony Loyd, a reporter for The Times of London, last month.

Speaking through an English translator, a spokesman for the Syrian Democratic Forces, Mustafa Bali, confirmed the death of the weeks-old infant on Friday, but did not provide details. Earlier in the day, Mr. Bali had tweeted that reports of the baby’s death were untrue, only to later delete the statement.