AMSTERDAM — For Aysha and Luca Opdam's father, it is still hard to accept that his children, born and raised in the Netherlands, are now living in Syria. In a letter, he describes how his ex-wife radicalized after she devoted her life to Islam around six years ago. After it was made public that Aysha, 7, and Luca, 8, were taken by their mother to join the Islamic State (IS), the letter was published by a local newspaper.

"If my kids — if I ever get them again — become Muslims that would be fine by me. But only at an age when they are able to make their own choices with full awareness,” he wrote on March 22.

The mother, a Chechen refugee, and their children disappeared on Oct. 27, 2014, from Maastricht, a city in the Netherlands, and arrived late December in Tell Abyad, a Syrian town near the Turkish border. They are now living in Raqqa, the self-proclaimed capital of IS. The father hasn’t heard anything from them for months.

The London-based International Centre for the Study of Radicalisation and Political Violence estimates that at least 4,000 Western citizens have joined the Syria-Iraq conflict. About 550 are women who have migrated to IS-controlled territories, according to the Institute for Strategic Dialogue. How many Western children have been taken remains unknown. Most of the women travel without children.