Amsterdam - Radical Islamist groups in the Netherlands have become a decentralised and elusive "swarm" that may broaden their focus from the conflict in Syria to the wider Middle East, the Dutch intelligence service warned on Monday.
Its report reflects widespread concern in Europe at the threat posed by European citizens - mainly from an Islamic immigrant milieu - leaving to fight in Middle East conflicts and returning battle-hardened and posing a security threat.
Dutch authorities estimate that 120 Dutch citizens have so far left to fight in Syria's civil war, but said that a larger movement of radical Muslims in the Netherlands had several hundred adherents and thousands of sympathisers.
The Dutch General Intelligence and Security Agency (AIVD) said in its latest assessment of the threat posed by underground jihadist groups that they were stronger and more self-confident.
The agency said the phenomenon was becoming ever harder to track as social media made it possible for increasingly "professionalised" radical movements to co-ordinate themselves without the need for a centralised authority.
"The movement has taken on the character of a swarm," the agency said. "There is a less hierarchical structure than at the turn of the millennium, which makes it more flexible, effective and less vulnerable to 'attack' from outside."
The Netherlands has banned recruiting for militant groups and is also weighing legal measures to prevent its nationals from joining foreign insurgencies.
An 18-year-old man was recently arrested in The Hague on suspicion of recruiting people to take part in the Syrian conflict, and some 30 people suspected of planning to do so have had their passport seized, the government's anti-terrorism co-ordinator said in a letter to parliament on Monday.
European governments have struggled to stop their nationals, some just teenagers, from travelling to Syria where the conflict, which began as peaceful protests against President Bashar Assad and evolved into an armed rebellion now in its fourth year. More than 160 000 people have been killed.
But the AIVD warned that radicals in the Netherlands might turn their attention towards fighting in other current or potential Middle East conflict zones.
"For now, Dutch jihadis are heavily focused on Syria, but that could change," its report said. "It could involve existing conflict zones likes Yemen and Iraq, but even potential new zones like Egypt - including Sinai, or Libya."
Would-be fighters in Middle East conflicts are growing more skilled at evading the attention of authorities, buying return tickets to the region and posing as tourists, it said.
"News from conflict zones in Syria is spread rapidly via chat, Facebook and email within hours or even in real time within a jihadi inner circle in the Netherlands," said the report. It added that the number of jihadist publications in Dutch had increased sharply over the past two years.
The French citizen suspected of shooting dead four people at the Jewish Museum in Brussels in May spent time fighting in Syria. Earlier this month, German intelligence said some 2 000 Europeans had gone to Syria to fight alongside Islamist rebels.