The infant son of Shamima Begum, a teenager who left London to join the Islamic State group in Syria, has died, a spokesman for the US-backed Syrian Democratic Forces (SDF) said on Friday.
Begum, whose first two children also died, was stripped of her British citizenship last month on security grounds after she was discovered in a detention camp in Syria.
The 19-year-old left London to join IS when she was 15. She had sought to return to Europe with her third child, who was born about three weeks ago.
SDF spokesman Mustafa Bali said the child had died. Begum has said in media interviews her son was named Jarrah.
Begum married Yago Riedijk, a Dutch fighter for IS who surrendered to Syrian fighters and was being held in a Kurdish detention centre in northeastern Syria.
A British government spokesman said: "The death of any child is tragic and deeply distressing for the family.
"The Foreign and Commonwealth Office has consistently advised against travel to Syria since April 2011. The government will continue to do whatever we can to prevent people from being drawn into terrorism and travelling to dangerous conflict zones."
The family's London-based lawyer on Friday said the baby's death had been confirmed, after saying earlier that there were strong but still unconfirmed reports of the infant's death.
The fate of Begum has illustrated the ethical, legal and security conundrum that governments face when dealing with the families of militants who swore to destroy the West.
The US-backed SDF is now trying to take Islamic State's last, small patch of ground in eastern Syria. They have slowed their offensive on the jihadist enclave at Baghouz near the Iraqi border to allow many thousands of people to pour out in an exodus that has lasted weeks.