Around a third of the estimated 5,000 European Islamist militants who went to Syria and Iraq have returned to Europe, and some may have orders to attack, an EU report warned Wednesday.
Up to 2,500 fighters from Europe remained on the battlefield but their massive return in the short term seemed unlikely, according to the report seen by AFP.
Belgium expressed concern last month that Islamist militants were increasingly returning to Europe as US-backed coalition forces drive the Islamic State (IS) militant group from territory in Syria and Iraq.
The report said between 15 to 20 percent of the Europeans have died on the battlefield, around 30 to 35 percent have returned and 50 percent remain in the battle theatre, which amounted to between 2,000 and 2,500 Europeans.
As many as 1,750 may have returned, based on the percentages listed in the report which EU counter-terrorism coordinator Gilles de Kerchove will present to EU interior ministers on Friday.
The report said there were two types of "foreign terrorist fighters" returning.
"Those in the majority that will drift back, and those who will be sent back on specific missions, which are of most concern," the report warned.
It said even some European women and children born or raised in the so-called caliphate declared by IS in Iraq and Syria could pose a security threat as they may have been radicalised.
Without giving figures, it said some returnees have been convicted and serving prison sentences, while others are being monitored and some are free in their communities.
It recalled that foreign fighters who have returned to Europe have staged both foiled and successful attacks, including the slaughter in Paris in November last year and this year's bombings in Brussels in March.
Both sets of attacks were claimed by IS, which is also known by its Arabic acronym Daesh.
"There is also a significant foreign terrorist fighter contingent with Daesh in Libya which might attempt to use their nationality or family connections to return to Europe," the report said.
It said returnees were keeping in touch with Daesh in the Middle East via social media and increasingly turning from mainstream Twitter to the encrypted one-to-one messaging service Telegram.