The EU's counter-terrorism chief Gilles de Kerchove has said that around 3,000 people from across Europe are now working alongside Islamic State (Isis) fighters, compared with about 2,000 just a few months ago.
Gilles de Kerchove told news agency AFP that at least eleven European countries including Denmark have seen citizens travel to Iraq and Syria.
Just over a month ago a Swedish terrorism expert - Magnus Ranstorp - estimated that between 300 and 350 Scandinavians had links to Isis.
On Tuesday, Gilles de Kerchove said that numbers may have been boosted by the Islamic State's declaration in June of a caliphate (an area ruled by a supreme religious and political leader) straddling the two countries.
"The flow has not dried up and therefore possibly the proclamation of the caliphate has had some impact," de Kerchove said.
His comments came hours after the United States and its Arab allies began strikes from land and sea on Islamist militants in Syria.
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The EU counter-terrorism chief said he would check his figures with those of European Security Service chiefs in the next couple of weeks, suggesting their numbers may be more conservative.
Gilles de Kerchove said that European fighters working with Isis mainly come from France, Britain, Germany, Belgium, The Netherlands, Sweden and Denmark. But he said some are now also travelling from Spain, Italy, Ireland and most recently Austria.
"I think even a country like Austria has foreign fighters now, which I was not aware of before," he added.
When measured by total population, Denmark has sent the second-highest number of foreign fighters to Syria, behind only Belgium. Danish authorities estimate that at least 100 Danes have fought in Syria. Denmark's outsized contribution of foreign fighters led the government to announce a new anti-jihadist strategy last week.
In Germany, growing numbers of teenagers are reported to be joining Isis, with at least 24 people under 18 known to have made the journey to Syria or Iraq.
Meanwhile, Martin Bernsen at PST, the Norwegian Security Police, added that it was aware of young people from Norway signing up.
"We have examples of minors who have entered or attempted to travel to Syria", reported Norwegian news agency NTB.