The Kurdish-led administration in northern Syria has called for the creation of an international tribunal to try thousands of suspected members of the Islamic State (IS) group.
One official, Abdul Karim Omar, told the BBC they were struggling to cope with the thousands who emerged from the last IS enclave of Baghuz, in the east.
Kurdish-led Syrian Democratic Forces (SDF) captured the village last week.
About 1,000 foreign fighters are among the thousands held by Kurds in prisons.
The men are said to come from some 50 countries.
US President Donald Trump hailed the capture of Baghuz although he said the US would "remain vigilant" as the group remains a threat.
At its height, IS controlled 88,000 sq km (34,000 sq miles) across Syria and Iraq.
While it no longer holds this territory, US officials believe IS may have 15,000 to 20,000 armed adherents active in the region, many of them in sleeper cells, and that it will return to its insurgent roots while attempting to rebuild.
What did the Kurds say?
In a statement, the Kurdish administration called for "a special international tribunal in north-east Syria to prosecute terrorists" to ensure that trials are "conducted fairly and in accordance with international law and human rights covenants and charters".
Speaking to the BBC, the administration's head of foreign affairs, Abdul Karim Omar, said the fact so few nations had repatriated their citizens who joined IS has added to their problems.
Many Western governments have refused to repatriate their citizens amid concerns over the potential security risks they may pose, as well as the challenges of gathering evidence to support prosecutions.
The US-backed SDF forces have reportedly captured more than 5,000 militants - from Syria and abroad - since January and put them in detention centres, while women and children are kept in camps for the displaced.
According to one Kurdish official speaking to AFP news agency, more than 9,000 foreign relatives of IS members - many of them children - are being held at the Kurdish-run al-Hol camp. The accommodation was built for about 20,000 people but is now housing more than 70,000.
By Aleem Maqbool, BBC News, north-eastern Syria
Meeting Kurdish officials, their frustration with Western countries becomes all too clear.
The administration's foreign affairs chief told me he felt the Kurds had been left to deal alone with the detention of IS members and their families with no plan in place as to what happens next. It has struggled to cope with even detaining the militants it has captured, let alone putting them on trial.
He said he had been hugely disappointed in countries who had revoked the nationalities of their citizens who had joined IS, saying the Kurds had already suffered so much loss in living under IS and then fighting the militant group.
His warnings were stark; that leaving dangerous members of IS in an unstable region held by an administration ill-equipped to process them was asking for trouble; and leaving children to remain surrounded by the harmful ideology into which they were born was storing up profound problems for the future.
What has been the reaction?
US envoy for Syria James Jeffrey told reporters on Monday the focus was on repatriating militants, and "getting countries to take back their own foreign terrorist fighters".
Iraqis and Syrians should be sent home "for deradicalisation and reintegration or in some cases punishment", while countries around the world should be encouraged to "take back their own citizens".
However, some have pointed to the case of Hoda Muthana, a woman from Alabama who left the US in 2014 to join IS and has now been refused permission to return to the US by Secretary of State Mike Pompeo, a decision backed by President Trump. Mr Pompeo says she was never a US citizen, but her family disputes this.
Asked if the US was considering an international tribunal, Mr Jeffrey said: "We're not looking at that right now."
Could an international tribunal work?
There have been several international tribunals in the past, including to try war crimes in the Balkan conflicts of the 1990s and after the Rwandan genocide in 1994. Since 2002, the International Criminal Court has served as a permanent international tribunal but the Rome Statute establishing it has not been ratified by Syria.
But transitional justice expert Joel Hubrecht told AFP that it was "not realistic" to set up such a tribunal in north-east Syria as:
- Syria's Kurdish authorities are not internationally recognised
- setting up such a tribunal usually takes time
- ensuring witness protection is difficult in a war-torn country