AMSTERDAM — The Netherlands raised its alert level for terrorist attacks to "substantial" on Wednesday, citing an increase in the number of Islamist militants traveling from the Netherlands to Syria, as well as a radicalization of Dutch youth.

"The chance of an attack in the Netherlands or against Dutch interests abroad has risen," the National Coordinator for Security and Counterterrorism (NCTV) said in a statement.

"Close to a hundred individuals have recently left the Netherlands for various countries in Africa and the Middle East, especially Syria."

The agency said individuals fighting for radical Islam abroad could return and "inspire others in the Netherlands to follow in their footsteps."

Political changes in the Middle East and North Africa have made space for an expansion of radical Islamic groups that are no longer under the control of security forces, the agency said.

Dutch police and intelligence services have deployed extra personnel to investigate suspect individuals and monitor sources, the agency said.

The Netherlands has not suffered a major terrorist attack, but a radical Dutchman of Moroccan origin murdered the provocative filmmaker Theo van Gogh, who was critical of multiculturalism and of Islam, in Amsterdam in 2004.

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