Germany and Finland have repatriated five women and 18 children from Syrian camps housing suspected relatives of ISIS fighters.
The foreign ministries of the two countries said that the 23 people brought back at the weekend on humanitarian grounds.
A chartered plane brought back three women and 12 children to Frankfurt Airport early on Sunday morning, before flying on to Helsinki to return eight Finnish nationals home. The joint operation between the two countries was kept secret until the returnees arrived in Germany.
German media reported that three of the women, aged between 21 and 38, were under investigation for allegedly belonging to the terrorist group. The the children are between two and 12 years of age, reports said. The group lived in a refugee camp under Kurdish control.
A spokesman for the Kurdish administration's foreign relations office, Kamal Akif, told the Agence France-Presse that the three women were the wives of ISIS militants and were "in very poor health".
One of the German women was detained on arrival in Frankfurt, the anti-terrorist prosecutors' office said.
"I am very relieved that we were able to repatriate another 12 children and three mothers from camps in north-east Syria yesterday," German Foreign Minister Heiko Maas said.
Mr Maas said the repatriates were mainly orphans and children who were ill, making their return "particularly necessary and urgent".
"Under the constitution, Finnish public authorities are obligated to safeguard the basic rights of the Finnish children interned in the camps insofar as this is possible," the Finnish foreign ministry said.
“The basic rights of the children interned in the Al Hol camps can be safeguarded only by repatriating them to Finland.”
The ministry has said that more than 9,000 foreign women and children, two thirds of them children, are being held in the Al Hol and Roj camps in north-east Syria, housing suspected ISIS militants and their relatives.
About100 German adults who are ISIS followers, plus a further 150 children, are currently languishing in camps in Syria under Kurdish control. They are waiting to be repatriated.
Advocacy group Human Rights Watch has described the conditions in the camps as “appalling”, but Berlin has mainly taken a tough line on returning their nationals from Syria. Sunday’s case was the first time Germany had been so active on bringing ISIS recruits home.
Many European countries have been reluctant to return their citizens who left the country to join ISIS because of security risks.
The UK for example has organised the return of a small number of orphans but has also revoked the citizenship of adults to prevent their extradition.