The number of Europeans joining Islamic State in Iraq and the Levant (ISIL) fighters 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 ISIL militants in Iraq since August and on Monday began targeting ISIL in Syria.
The UK parliament is due to vote on possible air strikes in Iraq on Friday. ISIL has seized large parts of Iraq and Syria in recent months.
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.
The CIA estimates that ISIL may have up to 31,000 fighters in Iraq and Syria - three times as many as previously feared.
De Kerchove said that ISIL'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.
"That was clear with the French because three days ago ISIL issued a statement saying there would be retaliation against the coalition. A French man was kidnapped in Algeria and he has been beheaded. So they did what they announced." De Kerchove also warned that groups competing with ISIL, such as al-Qaeda, may try to launch attacks in Europe to maintain their profile.
"The rise of ISIL may prompt al-Qaeda to do something to show that it is still relevant," he said.
On Wednesday, the UN Security Council adopted a binding resolution compelling states to prevent their nationals joining jihadists in Iraq and Syria.
The US military released footage of air strikes on ISIL-controlled oil refineries in eastern Syria on Thursday, carried out by US, Saudi and UAE aircraft.
Sales of smuggled crude oil have helped finance the jihadists' offensive in both countries.