Young, radicalised Muslims who are being prevented from leaving the Netherlands to fight with militias in Syria are forming an increasing domestic threat, counter-terrorism chief Dick Schoof has told broadcaster Nos.

‘We are not only worried about people coming back [from Syria and Iraq],’ he said. ‘It is also about people who are frustrated or inspired and start following violent extremism.’

The counter-terrorism focus in the Netherlands until now has been on returnees who have been trained in the use of guns and explosives and are often traumatised or brainwashed.

The Dutch security service AIVD is currently closely monitoring 33 Syria returnees, Schoof told Nos. ‘And in principle, they are all under observation.’

However, the dozens of youngsters who have been prevented in joining IS are also now being seen as a potential threat, he said.

Lawyers have already warned that the repressive government policy could have the reverse effect. ‘The effect of these measures is to make people angry about the Dutch government,’ lawyer Michiel Pestman said earlier this year.