Brussels (AFP) - Brussels struggled to return to normal Wednesday after four days on maximum terror alert, with schools and the metro reopening despite two suspects from the Paris attacks still being at large.

Troops and police were still patrolling the streets of the Belgian capital as the alert status remained at the highest possible level of four, leading to questions about what had changed since schools had closed on Monday.

"It's not very reassuring, is it?," said Sarah, who runs a private nursery in Brussels. "If it wasn't safe to open on Monday and Tuesday, why is it now?".

For two days parents scrambled to cope with the extraordinary decision to close schools which authorities said was necessary to foil an imminent Paris-style attack by jihadists.

A prosecutor on Wednesday decided to extend the detention of one suspect, 39-year-old Lazez Abraimi, leaving five people behind bars in Belgium in connection to the November 13 gun and bombing attacks in Paris.

Police found a firearm and traces of blood in Abraimi's car late last week and his lawyer said prosecutors suspected he helped top suspect Salah Abdeslam, who is still at large, escape a manhunt.

Belgium on Tuesday issued an international arrest warrant for another "dangerous" man, Brussels-based Mohamed Abrini, who was seen driving a car with Abdeslam on the eve of the attacks.

Interior Minister Jan Jambon said police carried out raids on Sunday on evidence that a similar bloodbath was imminent in Brussels.

"This was avoided and that is a good thing," Jambon said, without giving further details.

- Paratroopers outside school -

The Brussels metro system, which had been closed since Saturday, gradually returned to service but with many stops still shut to better deploy security staff.

Car traffic was also backed up more than usual in Brussels, home to the headquarters of NATO and the European Union and already one of the Europe's most congested cities.

About 300 extra police from other cities in Belgium were sent to Brussels to guard schools and 200 extra soldiers helped patrol the metro.

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In the neighbouring Flanders region, the public bus company offered a special 50-euro bonus to drivers who agreed to operate services to the capital.

"Thank goodness we only have a half day today," said 14-year-old Marc as he boarded a school bus, adding that he had fully enjoyed the two extra days off from school.

At the French Lycee in a posh Brussels suburb, paratroopers cradled automatic rifles at the top of the small street leading to the school.

"I'm not reassured... this school is a symbol," said Godelieve, who only gave one name, as she dropped off her children.

Across town, the huge Kinepolis cinema multiplex reopened with added security by local police. Public museums and concert halls also returned to normal, though French pop singer Veronique Sanson cancelled a performance.

Belgian Prime Minister Charles Michel on Monday extended the highest threat level by at least a week, but took the decision to reopen the schools from Wednesday.

School officials were under orders to implement strict guidelines, including not letting parents onto the premises.

"I had nightmares all night," said Fatima, whose child attends a kindergarten north of Brussels.

The increased security presence in front of the school was discreet, with police patrol cars moving between three schools in the neighbourhood.

"They drove by six times this morning," said the school principal as the morning bell rang, signalling that it was time for parents to leave.