BRUSSELS (AFP) - The European Commission was among possible targets for jihadist fighters returning from Syria, reports said Saturday, as the Belgian authorities cracked down in an effort to thwart extremist attacks.

Dutch public broadcaster NOS said at least two people among those arrested in the Belgian operations came from the The Hague.

"They were planning an attack. One of the targets was the European Commission building," NOS reported, citing unnamed sources.

"Individual commissioners were not particularly targeted. It would be more like the Jewish museum attack... with the aim to kill as many people as possible," it added.

The suspect in the attack on the Jewish museum in central Brussels in May, which left four people dead, is Frenchman Mehdi Nemmouche who 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".

"We are aware of the reports," a Commission spokesman told AFP when asked for comments. "We are confident the national authorities will ensure the appropriate follow-up."

The Commission is the public face of the European Union and one of the most high-profile buildings in Brussels, housing several thousand officials and the top brass in charge of the daily running of the 28-nation bloc.

Brussels is also home to the NATO headquarters and many other international companies and organisations but security is mostly low-key and discreet.

- 400 Belgian nationals in Syria -

Belgian daily L'Echo said earlier that the authorities in recent months had carried out pre-emptive operations against jihadists and symphathisers with the Islamic State extremist group, resulting in several arrests.

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."

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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.

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, adding that their efforts would continue.

In its report, NOS said a man and woman of Turkish descent were arrested early last month when they arrived in Brussels on a flight from Turkey. They had allegedly been in Syria.

Police in Brussels and The Hague simultaneously raided homes in the two cities including in The Hague's Schilderswijk district, scene of a recent pro-IS demonstration.

Belgian police found guns and bullet-proof vests while Dutch police uncovered jihadist literature, NOS said.

Belgium like many European countries is increasingly concerned about its nationals going to fight in Syria and Iraq for fear they will return home battle-hardened and even more radicalised, 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 organisation.