Up to 5,000 Europeans have returned to the continent after being trained at terrorist camps, the head of Europol has reportedly said.
Rob Wainwright claimed Europe is facing its biggest terror threat in more than 10 years and has warned of large-scale attacks by IS and other groups.
The head of the EU law enforcement agency said: "The growing number of foreign fighters presents the EU countries with completely new challenges."
At least 700 people from the UK have travelled to support or fight for jihadist groups in Syria and Iraq, according to police, and about half have since returned.
Mr Wainwright's comments to Germany's Neue Osnabrücker Zeitung newspaper come after November's Paris attacks in which 130 people were killed.
He claimed 3,000-5,000 Europeans had returned from terror training, and said further attacks in Europe were expected, with more civilian casualties.
He said: "It is expected that IS or other religious terrorist groups will carry out an attack somewhere in Europe, with the aim of achieving high losses among the civilian population."
He added that not only groups of militants, but also individuals could be expected to carry out strikes.
But the Europol boss claimed that attackers were not trying to use Europe's refugee crisis as a "systematic" cover to come in unnoticed.
He told the German newspaper: "There is no concrete evidence that terrorists are systematically using the flow of refugees to arrive undetected in Europe."
However, after the Paris attacks, the French Prime Minister Manuel Valls said some of the gunmen had exploited Europe's refugee crisis to "slip in" to the country unnoticed.