Brussels, Nov. 25 (Reuters): The Brussels metro and schools reopened today as the Belgian capital edged back to normal after a four-day security lockdown but the hunt went on for militants police fear may mount a Paris-style mass attack.

Troops and armoured vehicles outside reopened underground stations - about half of the network remains closed - and police outside schools provided a reminder of the threat the government said was so imminent that it raised its alert level on Saturday in Brussels to the maximum, where it remains.

There was less evidence of the raids and searches that have been carried out periodically in the city since two suicide bombers from Brussels blew themselves up on November 13 during the IS attack on Paris. The brother of one of them is still on the run, along with at least one suspected accomplice.

Of more than two dozen people detained in the past 12 days in Belgium, all but five have been released. Police have not reported finding substantial amounts of weaponry which the government says it fears a local extremist cell could be about to use in a rerun of the violence in France that killed 130 people.

"There are maybe 10 or more people in Belgium, maybe in the neighbouring countries, present in the territory to organise some terrorist attacks," foreign minister Didier Reynders, speaking English, told ABC television. Aides later said the figure was based on an assumption of how many people would have to be involved to mount attacks like those in Paris.

Sister newspapers L'Echo and De Tijd quoted unidentified sources saying security operations on Sunday had foiled a major attack in Brussels. Officials declined comment and judicial sources told public broadcaster RTBF that Sunday's raids, during which 16 people were detained, were not intended to thwart attacks but rather to "give the hornets' nest a good kick".

Of five persons held in Brussels on terrorism charges, two admit driving Salah Abdeslam, brother of suicide bomber Brahim Abdeslam, back from Paris to Brussels just after the attacks but deny any knowledge of the plot.

Salah Abdeslam, 26, is suspected of being the eighth attacker mentioned by IS in its claim of responsibility. The seven others died. A suicide vest found in a Paris litter bin and mobile phone records have suggested that he may have had a change of heart.