It was too dangerous to send British officials to rescue Shamima Begum's baby son in Syria, Foreign Secretary Jeremy Hunt has said.
The child died in a refugee camp after his mother, who joined IS in 2015, was stripped of UK citizenship.
The boy was a UK citizen - but Mr Hunt told the BBC that any rescuers' lives would have been at risk in the camp.
"The mother chose to leave a free country to join a terrorist organisation," he said.
Speaking on The Andrew Marr Show, the foreign secretary confirmed that Jarrah, who was three weeks old, was a British citizen even though his mother was not.
But he said that - although several journalists had reached the camp and spoken to Ms Begum - "we have to think about the safety of the British officials that I would send into that warzone".
"Shamima knew when she made the decision to join Daesh, she was going into a country where there was no embassy, there was no consular assistance, and I'm afraid those decisions, awful though it is, they do have consequences," he said.
He said that the Foreign Office and the Department for International Development were looking at ways to find the British children of other so-called "Islamic State brides" and get them out.
Details have emerged of two more women from the UK, who are in Syrian camps with their young children, who have been stripped of their citizenship.
Reema Iqbal and her sister, Zara, from east London, were first named by The Sunday Times, quoting legal sources.
Sources told the BBC that the decision to remove their citizenship was taken by the former Home Secretary Amber Rudd, who left office in April 2018.
The Home Office said it did not comment on individual cases. Decisions to withdraw citizenship from individuals were evidence-based and not taken lightly, it added.
The use of the powers has risen sharply, with 104 deprivations of citizenship in 2017, compared to 50 in the previous decade, according to Home Office figures obtained by the immigration law website Free Movement.
Many cases have involved national security and supporters of groups such as Al-Qaeda but criminals - including three of the Rochdale grooming gang - have also been stripped of citizenship.
The Sunday Times says that Reema, 30, and Zara, 28, are living in separate refugee camps in Syria - along with thousands of other families who have fled from territory formerly controlled by jihadis.
Between them they have five boys under the age of eight, it says.
The parents of the sisters are originally from Pakistan, but it is not known if they have dual nationality.
According to the Sunday Times, the sisters left for Syria in 2013 after marrying IS fighters with "close links" to the filmed murders of western hostages.
Zara was heavily pregnant with her second child when she travelled to Syria and later gave birth to a third. Reema has one son born in the UK and another born in Syria.
Home Secretary Sajid Javid has faced criticism for his handling of the similar case of Ms Begum.
Her three-week-old son, Jarrah, died of pneumonia on Thursday, according to a medical certificate.
Shadow home secretary Diane Abbott said the child had died as a result of the "callous and inhumane" decision to strip Ms Begum of her citizenship - while Tory MP and former justice minister Phillip Lee urged the government to "reflect" on its "moral responsibility" for the tragedy.
A UK government spokesman said: "The death of any child is tragic and deeply distressing for the family."