The Belgian authorities have prevented several attacks by jihadist fighters returning home from Syria and by sympathizers with the Islamic State extremist group, a report said Saturday.

The daily L’Echo cited unnamed sources as saying the planned attacks could have been similar to the one on the Jewish Museum in central Brussels in May which left four people dead.

The suspect in that case, Frenchman Medhi Nemmouche, spent more than a year fighting with Islamist extremists in Syria and is now being held in Belgium on charges of “murder in a terrorist context”.

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Up to 400 Belgian nationals are estimated to have gone to fight in Syria, with about 90 known to have returned home, L’Echo said.

“Our starting point is that among them, one out of nine aim to carry out an attack,” a source said. “That is a conservative estimate, if you also take into account the people who help them.”

The newspaper said its sources did not want to give details of the planned attacks nor of the operations to prevent them, for reasons of security and so as not to alarm the public.

Several arrests have been made, it added.

Officials declined to provide any details but did confirm that several operations had been carried out.

“We are working full time on the problem of the returning fighters,” Belga news agency cited a federal judicial spokesman as saying.

“We work together with the security services and that led us to carry out several operations,” the spokesman said.

Belgium like many European countries is increasingly concerned about nationals going to fight in Syria and Iraq for fear they will return home battle hardened and even more radicalized, posing a threat to security.

In June, a court ordered that 46 suspected members of Sharia4Belgium, a radical Islamist group believed to be involved in sending young fighters to Syria, should stand trial on various charges, including involvement in a terrorist organization.