A spokesperson for the U.S.-backed Syrian Democratic Forces (SDF) has issued a stark warning to the international community, saying those that trusted Turkey to detain Islamic State fighters would ultimately only have themselves to blame.
A Turkish offensive on Kurdish-held areas in northern Syria entered its third day on Friday.
It has sparked a global chorus of condemnation, with many deeply concerned the conflict has opened a new front in Syria's eight-year war.
In response, Turkish President Recep Tayyip Erdogan has since lashed out against those critical of his military operation.
On Thursday, Erdogan threatened to send millions of Syrian refugees to Europe if the continent's leaders described Turkey's invasion of Northern Syria as an "occupation."
Mustafa Bali, a spokesman for the SDF — an organization now tasked with governance of the area and containment of resurgent IS fighters and overcrowded IS prisons — had already warned global leaders to expect this from Erdogan.
He also urged the international community to expect the same type of threat when it comes to IS prisoners.
"One day, when Turkey uses captured ISIS fighters as a threat to Europe and the world just like it uses Syrian refugees now, we will remind those who trusted Turkey in managing detention of militants that their complicit silence was the main reason (that) caused it," Bali said via Twitter on Wednesday.
Turkey's Foreign Ministry could not be reached via telephone when contacted by CNBC on Friday.
Trump has insisted that Turkey must assume responsibility for the captured IS fighters and their families. But, the military's ability to continue securely holding some 11,000 prisoners in more than 30 detention centers remains highly uncertain.
"The question of Turkey threatening to send ISIS fighters back to Europe is probably not the key concern for Europe in the short term, in the sense that Turkey already has leverage over Europe by threatening to let millions of migrants flow into Europe," Adeline Van Houtte, an analyst at the Economist Intelligence Unit, told CNBC via email.
"However, the ISIS issue will become very preoccupying for the region and then Europe as Trump has handed Turkey responsibility for a high number of ISIS prisoners held in northern Syria."
On Wednesday, European Commission President Jean-Claude Juncker called for "Turkey, as well as other actors, to act with restraint."
Erdogan has pledged to clear the area of "terrorists," and says his aim is to allow a path for the return of Syrian refugees in Turkey to go back home. Numerous U.S. officials have cast doubt on that promise.
Ankara has long vowed to wipe out the Kurdish militia presence along its border in northern Syria, which it views as a security threat and indistinguishable from a separate Kurdish terrorist group that is waging a counterinsurgency inside Turkey.
On top of a lack of desire to manage IS fighters currently held by the SDF, "Turkey's capacity to handle the situation is also most probably lacking," Van Houtte said.
Turkey hosts the largest number of refugees in the world, with 3.6 million registered Syrian nationals in 2018 and 40,000 refugees and asylum-seekers of other nationalities, according to the U.N. High Commissioner for Refugees.
— CNBC's Natasha Turak contributed to this report.