The Dutch government has raised its terror warning to the second highest level, saying there's a "substantial" possibility of an attack.
Dutch Muslims have been traveling to Syria to fight in the civil war. The top Dutch counterterror official said those jihadi fighters could return battle-hardened and more radicalized.
"The chance of an attack in the Netherlands or against Dutch interests abroad has risen," Dutch National Coordinator for Security and Counterterrorism Dick Schoof said in a statement.
Schoof said nearly 100 people had traveled from the Netherlands to Africa and the Middle East, mainly to Syria to fight in conflicts there. He warned it is not just a Dutch problem.
"From Europe as a whole, hundreds have made the journey, many of whom are joining local armed groups," the counterterror watchdog said.
"These jihadist travelers can return to the Netherlands highly radicalized, traumatized, and with a strong desire to commit violence, thus posing a significant threat to this country," Schoof added.
Last month, France also expressed concerns about its citizens heading to Mali to join radical Islamic fighters there, even as the French army was fighting the Muslim rebels in its former colony.
The Dutch government also sees signs of increasing radicalization among Dutch youth at home.