Belgian intelligence services, responsible for protecting the EU and Nato, have launched a major clear-up amid fears of Russian and other infiltration.

Belgium's intelligence oversight body, the so-called Comite R, opened an investigation into some 20 cases a few months ago, Belgian newspaper Die Tijd reported on Saturday (25 January).

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About 15 were in the military intelligence service, the ADIV, and five more were in its homeland service, the VSSE, Die Tijd said.

The VSSE denied that there were any cases in its ranks, however. Die Tijd's information was false, the Belgian homeland service told EUobserver.

Some of the cases concerned Russia, while some concerned other foreign powers, Die Tijd also said.

One was a former ADIV officer who owned a VIP jet service firm with a Russian major and a suspected Chechen mobster, EUobserver learned from a Belgian security source.

In a sign of the scale of the Comite R clean-up, the two Belgian services have just 1,300 or so personnel combined.

They form a thin blue line, protecting EU and Nato buildings and the 47,000 or so EU and Nato staff who live in Belgium from foreign espionage on top of their national security work.

And Brussels plays host to hundreds of Russian and Chinese spies, the EU institutions have in the past warned.

The Belgian military intelligence chief, Claude Van de Voorde, told Die Tijd in a rare public statement that the investigation would not be easy.

It was hard to find a "smoking gun" and the last time Belgium had successfully convicted a traitor in court was in the 1980s, the ADIV chief noted.

"We can confirm there is not one case of real infiltration known to our service," a spokeswoman for the VSSE added.

The Comite R will also look more broadly at how the ADIV and VSSE can better defend themselves in future.

It is led by a magistrate and has 16 officials and six investigators. It answers to parliament but is to give any evidence of wrongdoing it might find to courts.

News of the sweep comes after a VSSE whistleblower recently accused its head of counter-intelligence of being worryingly soft on Russia.

A Comite R investigator also threatened him and his family when he raised the matter internally, the whistleblower told EUobserver.

And the sweep comes after the ADIV's counter-intelligence chief was suspended last year for mishandling secret files in another sign of wider problems.

Belgian security services protect EU and Nato institutions situated in the country under old bilateral agreements.

"We have jurisdiction within our premises and the Belgian government has jurisdiction outside. We therefore cooperate closely on all aspects of security," an EU Council spokesman previously told EUobserver.

The "safety and protection" of Nato facilities was "ultimately assured" by Belgian services, a Nato spokesman said.

But for British intelligence expert Mark Galeotti, from University College London, Belgium has "shockingly" underinvested in its international duties.

"Talking to officials in Europe, I've often encountered concern about Brussels as a weak link for the EU and Nato. It's got that reputation," Galeotti told EUobserver in a previous interview.