Recent military successes by the Islamic State of Iraq and the Levant (ISIL) will encourage a new wave of European jihadist fighters to join the conflict, the Dutch government said Monday.

"I'm concerned that ISIL has become an important group to join for jihadist fighters from the Netherlands and Europe," said Dick Schoof, who heads the Dutch government's anti-terror agency NCTV.

He was referring to the stunning success of a recent offensive by Islamist militants from ISIS across Sunni areas of northern Iraq that has left Iraqi army forces in disarray.

"The success of ISIL will no doubt give a new impulse to travel there," Schoof told Dutch state broadcaster NOS.

The Dutch anti-terror tsar was speaking after Foreign Minister Frans Timmermans sent a letter to parliament on Monday in which he said "many of the Dutch jihadist fighters (in Syria) have joined up with ISIL."

More than 100 young Dutch citizens have so far travelled to Syria, which neighbours Iraq, to fight against President Bashar al-Assad's forces in the three-year conflict in which more than 162,000 people have died, according to NGO estimates.

At least 10 Dutch fighters have been killed in Syria so far, and they have carried out at least one suicide attack in Syria and one in Iraq, Dutch officials have said.

The European Union warned earlier this year that the number of young European Muslims going to fight alongside extremist groups in Syria and countries such as Somalia and Sudan was growing fast.

The fear is they will return home radicalised and well versed in the use of weapons and guerrilla tactics, posing a security risk.

Dutch police last month arrested a 21-year-old man who fought in Syria as he was allegedly about to commit a heist to finance fighting in Syria.

The Dutch authorities have said they will confiscate the passports of citizens they believe are wanting to go to the Middle East to fight.

Last week, the Dutch government said students opting to go to Syria would also forfeit their grants, as the government seeks to prevent Dutch citizens from travelling to Syria.