The number of people killed in attacks by extremists throughout Europe soared in 2015 from a year earlier, the European Union police agency reports as it warns that the Islamic State group may "put more emphasis on operations abroad" as a Western military alliance puts it under pressure in Syria and Iraq.

The Europol report paints a worrying picture of an EU assailed by Islamic extremist threats that are unlikely to recede any time soon. It warns that Syrian asylum-seekers could be target...