The U.S.-backed Iraqi operation to retake the Islamic State hub of Mosul could send thousands of fighters who traveled from Europe back home -- a process made easier as ISIS forges travel documents, European officials warned Sunday.
As many as 5,000 Europeans traveled to Iraq and Syria in recent years to join the terror fight, The Washington Post reported, adding that most remain on the ground there now.
"Further military losses, further military pressure on them in the region, indeed might lead to an increased reflex response by the group in Europe," Europol Director Rob Wainwright told the newspaper. He said European police had not yet detected "high numbers" of returning fighters, but added, "maybe Mosul and Raqqa could change that."
Military officials have said the Syrian ISIS hub of Raqqa represents a future target after the Mosul operation, which launched last week.
U.S. intelligence has found ISIS creating fake passports and arranging with organized crime groups for other forged documents to jump across borders.
Anti-terror officials in Europe have stepped up security at major airports across the continent. They've pointed out how many ISIS militants have posed as refugees to enter Europe, in some cases shaving their beards.
The Turkish military took steps last month to seal the nation's border with Syria, closing off what had served as a popular route for refugees, but European officials said it wasn't nearly enough to stop the terror threat. Europol reportedly sent more agents to keep watch on refugee flows into Italy, concerned some fighters could stop in Libya or elsewhere in North Africa before reaching Europe.
"The most important and pressing thing is to have [European] member states to be ready," a counterterrorism official told The Wall Street Journal.
At the same time, officials in France started booting thousands of refugees from a squalid camp known as "The Jungle" on Monday. The first of hundreds of buses began transferring migrants from the camp in Calais to reception centers around France where they could apply for asylum. Officials said workers will then destroy the camp in a weeklong operation.
Authorities say the camp holds nearly 6,500 migrants who are seeking to get to Britain. Aid groups say there are more than 8,300. France also remains in a state of emergency after the Paris terror attacks that killed 130 people in November of 2015.
The U.N. refugee agency reports at least 3,900 people, or about 650 families, have escaped Mosul and nearby al-Hamdaniya districts since the anti-ISIS operation began. Mosul is home to more than 1 million people.
The Associated Press contributed to this report.