BRUSSELS (REUTERS/AFP) - 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 heightened fears across Europe of young local Muslims returning radicalised from Syria.

But prosecutors' spokesman Eric Van Der Sypt said the Belgian probe had been under way before the Jan 7 attack on French satirical newspaper Charlie Hebdo.

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, Mr Van Der Sypt said, adding that they 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," he said.

"For the time being, there is no connection with the attacks in Paris."

Describing events in the quiet provincial town just after dark, he 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."

Earlier in the day, prosecutors said they had 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.

After the violence in Verviers, La Meuse newspaper quoted an unidentified police officer saying: "We've averted a Belgian Charlie Hebdo."

Two French brothers, who like Coulibaly claimed allegiance to Islamist militants in the Middle East, killed 12 people at the offices of Charlie Hebdo.

ISLAMIST STRENGTH

Belgium has seen significant radical Islamist activity among its Muslim population.

Public television RTBF showed video of a building at night lit up by flames, with the sound of shots being fired.

Marie-Laure from Verviers told RTBF she was in the street with her children when a police commando told them to run for cover.

"When we began running, we heard three or four big explosions and shots," she said. "It was really startling."

Another local resident said "machineguns were firing for about 10 minutes."

A third witness said he saw two young men apparently of North African origin "dressed all in black carrying a bag of the same colour," adding that the pair looked terrified.

Three Islamic State militants threatened attacks on Belgium in a video broadcast on Wednesday, the Belga news agency reported.

Belgian investigators said earlier on Thursday they were probing whether an arms dealer sold weapons used in the Paris attacks, after confirming supermarket gunman Amedy Coulibaly sold the man a car belonging to his partner Hayat Boumeddiene.

There was no immediate confirmation of any link between the Coulibaly investigation and Thursday's raid.

The man, Neetin Karasular, from the airport city of Charleroi in French-speaking southern Belgium, is in detention on suspicion of a possible link to the weapons used in the Paris attacks.

"The issue of weapons is under investigation," Mr Van der Sijpt told AFP earlier in the day, adding that Karasular was under suspicion for "arms trafficking Belgian prosecutors are working with French authorities to establish any "possible link" to last week's Paris attacks.

Coulibaly, who was killed by police on Friday, is also believed to have shot dead a policewoman in another Paris attack.

Mr Van der Sijpt added that the Belgian suspect "bought the car belonging to Coulibaly's wife."

Karasular handed himself into police on Tuesday, saying he had been in contact with Coulibaly in recent months and had tried to "swindle" the Frenchman over the car deal, but was scared after the Paris attacks.

Investigators searched his house and found documents proving the sale of the vehicle and papers showing negotiations with Coulibaly about arms and ammunition, including a Tokarev pistol of the sort used by the Frenchman during the supermarket attack, Belga said.

Karasular will appear before a magistrate in Charleroi on Monday who will decide whether he will remain in custody.

Spain, meanwhile, opened an investigation Thursday into Coulibaly and Boumeddiene's visit to Madrid shortly before the attacks.

Turkish authorities say Boumeddiene crossed into Syria on Jan 8 from Turkey.

She had arrived in Istanbul on a flight from Madrid before the Paris attacks took place.

Per head of population, Belgium is the European country from where the highest number of citizens have taken part in fighting with Syrian rebels in the past four years, data compiled by security researchers has shown.

Belgium has taken a lead in EU efforts to counter the threat perceived from the return of "foreign fighters" from Syria.

The Belgian government believes about 100 of its nationals have come back from there, while a further 40 may have been killed and about 170 are still in the ranks of fighters in Syria and Iraq.

Belgium is part of the US-led coalition fighting Islamic State in Iraq and Syria (ISIS) and has six F-16 aircraft taking part in bombing raids on Syria and Iraq.

A court in Antwerp is due to deliver its verdict on 46 people accused of recruiting young men to join militants or of becoming militants in Syria - Belgium's largest Islamist militant trial to date.

The court was to have given its verdict this week, but it was delayed for a month after the Paris violence.

In Germany, police arrested a suspected supporter of the insurgent group Islamic State who was recently in Syria, federal prosecutors said.

The 26-year-old German-Tunisian suspect, named as Ayub B, is thought to have entered Syria via Turkey last year to be trained in ISIS combat and recruiting, before returning to Germany in August, the German prosecutor said in a statement.

The German authorities have not uncovered any evidence of concrete preparations for an attack on their soil. Berlin estimates that some 550 German nationals have gone to fight in ISIS ranks in Syria and Iraq.