Belgian police launched a “jihadist-related” anti-terrorism operation in the eastern town of Verviers on Friday, with reports saying three people had been killed.
Public broadcaster RTBF reported three deaths and said explosions were heard at the scene, but there was no immediate confirmation.
The incident comes as Europe is on high alert after 17 people were killed in the Islamist attacks on the Charlie Hebdo magazine and a Jewish supermarket in Paris last week.
“An operation is under way,” a source in the mayor’s office told AFP without giving further details.
Another official said separately that the incident in Verviers, which is close to the German border some 125 kilometres (70 miles) from Brussels, was “jihadist-related”.
In May 2014, four people were shot dead in a suspected Islamist attack at the Jewish Museum in Brussels. Frenchman Mehdi Nemmouche, who had previously been in Syria, has been charged with murder.
The men targeted in Verviers were under surveillance having returning from Syria a week ago, Belgian media reported.
Intelligence indicated they were planning an attack, the reports said.
Several reports said a series of other anti-terror raids were under way across Belgium, including in the capital Brussels, where the European Union is headquartered.
Residents reported gunfire and blasts in the raid on a former bakery in central Verviers.
“I heard a sort of explosion, followed by several gunshots,” one local resident told RTBF. “For the moment, I cannot tell you any more because I don’t dare go out to see what is happening.”
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 yesterday, the Belga news agency reported.