The number of Europeans joining Islamist fighters in Syria and Iraq has risen to more than 3,000, the EU’s anti-terrorism chief has told the BBC.
Gilles de Kerchove also warned that Western air strikes would increase the risk of retaliatory attacks in Europe.
US-led forces have launched nearly 200 air strikes against Islamic State (IS) militants in Iraq since August and on Monday began targeting IS in Syria.
The UK parliament is due to vote on possible air strikes in Iraq on Friday.
IS has seized large parts of Iraq and Syria in recent months.
Mr de Kerchove said the number of 3,000 included all those who have been to the region, including those who have returned and those who have been killed there.