Anti-terror raids in Belgium appear to confirm long-standing fears the country has become a jihadist centre, with an often disaffected Muslim minority providing fertile ground for radicalisation.

Belgium estimates that 335 of its people have gone to fight in Syria and Iraq in the last few years -- putting it top of the list of European nations in proportion to its small population of 11 million.

The danger posed by returning fighters became clear Thursday when police killed two suspected militants and arrested 13 in raids on a cell of suspected Syria veterans who were allegedly planning attacks.

Claude Moniquet, CEO of the Brussels-based European Strategic Intelligence and Security Centre, said the raids "probably helped eliminate the threat in the short-term but it's not over."

The underlying problem in Belgium is well known -- a relatively large population of young Muslims without a job or a future who seek redemption in Syria or Iraq with groups such as the Islamic State.

The fear is that they return home fully-fledged fighters like those who carried out the Paris attacks which left 17 dead. The three men responsible for the Paris attacks, all French citizens, are believed to have had links with various jihadist groups in Yemen and Syria.

Of the 335 who have gone to fight from Belgium, some 184 are still there and 50 have been killed, while 101 have returned to Belgium.

Up to 5,000 European citizens in all may have gone to the Middle East's battlefields so far.

Researcher Montasser AlDe'emeh, currently studying why young Belgians fight overseas, said the fallout from the Paris attacks makes matters worse.

"Immediately after the attacks in Paris you have seen an increase in polarisation and fear," he told the Flemish language daily De Morgen.

The Belgian police raids "will only reinforce this", he added.

- Jewish museum attack -

Hans Bonte, the mayor of Vilvoorde, a suburb of northern Brussels, told AFP late last year that one reason Belgium had produced so many jihadists was because of a recruitment efforts by a group known as Sharia4Belgium.

Dozens of members of the group, which was officially dissolved two years ago, are now on trial for recruiting fighters to go to Syria. A verdict is due next month.

The group threatened to attack symbolic targets such as Belgium's royal palace and called for an Islamic state in Belgium.

In fact, Belgium was one of the first European Union countries to raise the alarm on the threat of "foreign fighters", pressing for more intelligence sharing and monitoring of suspects.

Progress has been painfully slow, however, even after the May killing of four people at the Jewish Museum in Brussels, blamed on Frenchman Medhi Nemmouche, who had come back from Syria.

The Paris attacks, however, have now galvanised opinion, with the EU set to hold a special summit next month dedicated to working out a comprehensive response.

The answer will not be straightforward.

"The motivations of the foreign fighters (who return to Belgium) are difficult to assess," said AlDe'emeh.

"Do they miss friends and family at home, were they disillusioned by what they saw in Syria or because they want to bring the fight to Belgium?

"No one can tell. People have left for as many reasons as they have found to come back."