The two suspected Islamic militants killed in a shoot-out with police in Belgium's Verviers town were just hours away from a terror attack targeting police installations in the town.

"They were on the verge of committing important terror attacks," said federal magistrate Eric Van der Sypt in Brussels.

Belgian authorities were just in time to thwart the attack when they killed two suspects in the shoot-out and arrested a third in massive anti-terror raids that began early in the evening and stretched into the night.

Security forces were acting on information that a terrorist cell comprising of people who recently returned from the Syrian battlefield were plotting an imminent strike.

They will continue to look for more clues in a weeks-long investigation that started well before the Paris deaths.

Like the Hebdo terrorists, the Verviers suspects too "were extremely well-armed men" equipped with automatic weapons.

Around 300 Belgian residents have joined ranks with extremist Islamic formations in Syria. It is unclear how many have returned.

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