Islamic State crisis: 'More than 3,000' Europeans join IS
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.
Earlier this year, experts put the figure at about 2,000.
Mr de Kerchove said that Islamic State's declaration of a Caliphate in June may have played a role in drawing more support from Europe.
"If you believe in this, probably you want to be part of it as early as possible," he said.
He warned that air strikes by the US and its Western allies had increased the risk of a violent response from militant Islamists against European targets.
"I think we have to acknowledge that it will," he said.