Europe's judicial cooperation agency, Eurojust, called for charging ISIS foreign militants returning home from the conflict in Iraq and Syria with war crimes.
It said many of these suspects currently only face charges under domestic terrorism laws.
However, they could face wider charges under international laws covering some of the worst crimes that happen during conflict, including genocide and crimes against humanity, said the network which was set up by the European Union to coordinate between national investigators and prosecutors.
"ISIS should not only be considered as a terrorist organization," Eurojust said in a new report.
The militant group could be treated under international humanitarian law as a "party to a non-international armed conflict in Iraq and Syria acting as an organized non-state armed group," it noted.
"Therefore, its members and foreign terrorist fighters could be responsible for committing war crimes and other core international crimes."
According to the report, examples of returned foreign fighters being prosecuted under cumulative charges already exist in France, Germany and the Netherlands.
"Prosecuting terrorism offences combined with acts of core international crimes ensures the full criminal responsibility of perpetrators, results in higher sentences and delivers more justice for victims, the report noted.
"By recognizing and naming these crimes for what they are, justice can hence be brought to victims," it stressed.
