The EU's counter-terrorism chief Gilles de Kerchove has said that around 3,000 people from across Europe are now working alongside 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 Sweden have seen citizens travel to Iraq and Syria.
He said that numbers may have been boosted by the Isis 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.
Just over a month ago a Swedish terrorism expert - Magnus Ranstorp - estimated that between 300 and 350 Scandinavians had links to Isis.
On Wednesday the Swedish National Defense College representative told The Local that the average age of fighters who have left Sweden to join the group is 21.
"That means that there are of course younger ones too who are 18 or 19, that is common," said Ranstorp.
In Germany, growing numbers of teenagers are also reported to be joining Isis, with at least 24 people under 18 known to have made the journey to Syria or Iraq.
Magnus Ranstorp is currently in Brussels where he is heading up a working group that is researching foreign fighters for the EU's Radicalization Awareness Network.
"We have been working hard to help affected cities across Europe, by seeing what we can do locally to stop people from signing up and how local authorities can help re-integrate fighters who have returned home," he told The Local.
"But it is a big challenge, " he added.
He hopes that former Social Democrat leader Mona Sahlin's recent appointment as Sweden's national coordinator against violent extremism will help improve communication between cities, organisations and agencies dealing with Isis recruits.
"The problem is that many people signing up are from Sweden's poorer suburbs and their decision is linked to other factors such as housing problems, difficulties securing jobs - even at a low level - and territorial gangs."
Most Isis fighters from Sweden have so far been recruited from Gothenburg, where at least 50 are known to have signed up.
Islamic militants are also known to have travelled abroad from Stockholm and Malmö as well as smaller towns such as Norköpping and Linköping in eastern Sweden. Magnus Ranstorp told The Local he is also aware of Islamic militants living in the north of the country.
The US has begun air strikes in Syria. Photo: Uncredited. Source: TT
Gilles de Kerchove's comments about the Europe-wide increase in Isis fighters 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.
But he added that he understood that European fighters working with Isis mostly come from France, Britain, Germany, Belgium, The Netherlands, Sweden and Denmark. 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.
Meanwhile, Martin Bernsen at PST, the Norwegian Security Police, added that it was also 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.