The EU's anti-terrorism chief says the number of Europeans joining militants in Syria and Iraq has risen to more than 3,000, according to 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 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.
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.
De Kerchove said that IS'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.
> Europe at Risk
He warned that air strikes by the US and its Western allies had increased the risk of a violent response from militants against European targets.
"I think we have to acknowledge that it will," he said.
"That was clear with the French because three days ago [IS] 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 IS, such as al-Qaeda, may try to launch attacks in Europe to maintain their profile.
"The rise of [IS] 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 extremist militants in Iraq and Syria.
> Subway Attacks Warning
Meanwhile, the Iraqi prime minister said on Thursday that Iraq has "credible" intelligence that IS militants plan to attack subway systems in Paris and the United States.
Prime Minister Haider al-Abadi's comments were met with surprise by security, intelligence and transit officials in both countries. New York's leaders scrambled to ride the subway to reassure the public that the nation's largest city was safe.
Abadi said he received the information Thursday morning from militants captured in Iraq and concluded it was credible after requesting further details. The attacks, he said, were plotted from inside Iraq, Reuters reported.
"They plan to have attacks in the metros of Paris and the US," Abadi told a small group of US reporters while in New York for the annual meeting of the UN General Assembly. "I asked for more credible information. I asked for names. I asked for details, for cities, you know, dates. And from the details I have received, yes, it looks credible."
Abadi did not provide further details. A senior Iraqi official traveling with him later said Iraqi intelligence had uncovered "serious threats" and had shared this information with its allies' intelligence agencies.
