Belgian special forces police block a street in central Verviers, a town between Liege and the German border, in the east of Belgium January 15, 2015. [Photo/Agencies]

VERVIERS, Belgium - Belgian police killed two men who opened fire on them during one of about a dozen raids on Thursday against an Islamist group that federal prosecutors said was about to launch "terrorist attacks on a grand scale".

Coming a week after Islamist gunmen killed 17 people in Paris, the incident fuelled fears across Europe of young Muslims returning radicalised from Syria. But the Belgian probe had been under way before the Jan 7 attack on French satirical newspaper Charlie Hebdo and officials saw no obvious link between the two.

A third man was detained in the eastern city of Verviers, where police commandos ran into a hail of gunfire after trying to gain entry to an apartment above a town centre bakery. All three were citizens of Belgium, which has one of the biggest concentrations of European Islamists fighting in Syria.

Other raids on the homes of men returned from the civil war there were conducted across the country, notably in several districts of the capital Brussels, prosecutors said. They added that the men were suspected of planning attacks on Belgian police stations. Security had been tightened at such sites.

"The searches were carried out as part of an investigation into an operational cell, some of whose members had returned from Syria," said prosecutors' spokesman Eric Van Der Sypt. "For the time being, there is no connection with the attacks in Paris."

Earlier in the day, in an apparently unrelated development, police detained a man in southern Belgium whom they suspected of supplying weaponry to Amedy Coulibaly, killer of four people at a Paris Jewish grocery after the Charlie Hebdo attack.

Belgian media quoted a national lawmaker as saying phone taps had prompted the operation. There has been concern in Europe that the French attacks, carried out by known radicals not seen as priority threats by security forces, might cause other groups to capitalise on public anxiety by accelerating plans to act.

Describing events in provincial Verviers after dark, Van Der Sypt said: "The suspects immediately and for several minutes opened fire with military weaponry and handguns on the special units of the federal police before they were neutralised."