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 fromSyria," 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.