Iraq has rejected Syria’s request to repatriate thousands of suspected Islamic State members held in Iraqi custody.

Iraqi National Security Adviser Qasim al-Aboudi said a meeting was held between Iraq, Syria and the United States to discuss the detainees’ fate. Damascus asked Baghdad to hand over about 4,000 Syrian prisoners.

‘Iraq is conducting investigations, and anyone proved to have committed crimes in Iraq will be tried under Iraqi law,’ al-Aboudi told the Associated Press.

Investigators found that about 400 of the detainees were not Islamic State members and can be released. About 50 others have already been handed over to Syria.

The prisoners were transferred from Syrian detention centers to Iraq earlier this year after fighting between Syrian forces and Kurdish-led groups raised fears of mass escapes.

Hundreds of those still held are believed to be high-ranking members of the group, officials said.

The detainee question has haunted the region for years. Thousands of foreign fighters and their families remain in limbo.

Western countries have resisted taking back their nationals. Iraq says it will not become a permanent jail for the world’s suspected militants.