Numerous foreign nationals with links to ISIS have come to Turkey since the terror group was defeated in Syria and Spain.
Former foreign fighters of the so-called Islamic State terror group (SIS) might still pose a threat in Turkey, a new report has found.
The International Crisis Group study -- published this week -- said ex-ISIS militants were an "enduring challenge" in Turkey, presenting officials with "complex questions".
"Thousands of foreign nationals who joined ISIS in Syria and Iraq have crossed into Turkey. Some have been deported. Others have stayed," the report said.
"Their presence creates a humanitarian and security challenge for Turkey, currently reeling from the devastating earthquakes of February 2023."
Around 41,000 people are estimated to have joined ISIS in Syria and Iraq between 2013 and 2018, according to King's College's International Centre for the Study of Radicalisation.
They came from more than 100 countries, including many European states.
"Turkey's location made it central to ISIS logistics and financing in the group’s heyday, and the presence of large numbers of ISIS-linked individuals still presents risks," said the report, which was written in Brussels and Ankara.
The terror group has since been militarily defeated, but there are thorny issues about what to do with those who were captured and imprisoned.
Many ex-fighters jailed in Turkey on ISIS-related charges are also facing release in near term.
Turkey has deported more than 9,000 foreigners since 2011, according to the report. However, many countries do not want to accept returned fighters, worried about security risks, legal problems and public opinion.
In the UK, the case of Shamima Begum, who was smuggled to Syria aged 15 and married to an ISIS fighter, has caused controversy, with London stripping her of British citizenship.
"The best outcome for ISIS-linked individuals is repatriation, provided it is to countries that treat returnees humanely," said the International Crisis Group.
Many ex-fighters meanwhile fear what will happen to them back home, fearing the reaction of society and the authorities.
Individuals who are likely to face persecution, torture or death on return are shielded by international law and must either be removed to a safe third country or allowed to stay in Turkey.
For those who cannot be deported, Ankara should consider "security measures" and "social programmes", it continued, suggesting foreign donors should support this.
"There is no single solution to the challenge Ankara faces. It will need to rely on a multi-pronged strategy that looks to policing and surveillance to track threats, while quietly tolerating (or even supporting) the integration of individuals who cannot be sent home."
The report called on foreign states to work with Turkey to improve cooperation and repatriate their nationals remaining in Turkey.
"The danger posed by ISIS has receded since it held swathes of territory in Iraq and Syria and launched attacks in the heart of Europe, but it has not disappeared entirely," it added.