There is no overall official figure available regarding EU citizens travelling to take part in the conflict in Syria, but estimates suggest that, by the end of 2013, they numbered between 1200 and 2000, Europol says.
Although depending on the developments in Syria, this number might possibly increase during 2014, the Hague-based European police body noted on Thursday.
Europol believes that the Turkish-Syrian border's accessibility is one factor why more European volunteers travelled to Syria rather than to Afghanistan, Mali, Somalia or Yemen.
It seems likely that Syria will remain the destination of choice for prospective militants departing from EU Member States, as long as the civil war there continues, said the report.
"In the wake of the Syrian conflict, the threat to the EU is likely to increase exponentially. European militants, who travel to conflict zones, are assessed as posing an increased threat to all EU Member States on their return," warned the report.
Between June and September 2013, nine individuals were detained in Ceuta (Spain), suspected of belonging to a network dedicated to sending volunteers to Syria, to fight alongside terrorist groups including al-Nusra Front and the Islamic State in Iraq and the Levant (ISIL), it noted.
The network appears to have had international links to Morocco, Belgium, Turkey and Syria, and to have successfully sent at least 12 young Spanish and Moroccan men to Syria, a number of whom died there in suicide attacks or combat, it said.
In addition, there were a number of arrests and convictions across Europe, including in Belgium, France, Germany, the Netherlands and the UK, in connection to travelling to Syria to participate in the conflict, it added.