Schools and some subway stations in the Belgian capital have reopened for the first time since emergency measures were imposed four days ago in the wake of the Paris attacks.

As police armed with automatic weapons stood guard, around half of all subway stations in Brussels were operating for the first time since the weekend.

Despite the easing of restrictions, Brussels - home to the European Union and Nato headquarters - remained under the highest alert level due to what authorities have described as a serious and imminent threat of attack.

Emergency measures had shut down shops, schools and subways in the city since Saturday.

The threat alert level is expected to remain at that level until at least Monday, barring a major development such as the arrest of suspects in the attacks.

The Belgian government has also ordered health and emergency services to take precautionary measures to ensure their services are not infiltrated by extremists.

"We have to be sure that we can see everybody has an identification badge," Health Minister Maggie De Block told VRT network.

"When ambulances arrive, we have to see from where they come, who is in it. Really as a precaution."

Belgian interior minister Jan Jambon said raids carried out on Sunday night had been designed to foil an imminent attack in Brussels.

Authorities had detained 16 people, but released all but one of them the following day. No explosives or firearms were seized.

Mr Jambon said "there were indications that there would be attacks on Sunday evening and they did not materialise", adding that otherwise "you don't impose terror level four", the highest possible.

The minister refused to elaborate what kind of attacks the government believed had been planned.

The reopening of schools and subway stations restored a sense of normality to Brussels, parts of which have been deserted for days.

At College Saint-Jean-Berchmans, some parents gave their children a quick kiss before dropping them off, while several police officers guarded the entrance, including one with a machine gun.

Among the students at the school is Belgium's Princess Eleonore.

Some children looked visibly worried as they arrived, but most gave a friendly handshake to a burly school official guarding the entrance alongside the police officers.

Authorities in France and Belgium have issued public appeals for help in tracking down two men believed directly linked to the Paris attacks that killed 130 people and wounded hundreds on November 13.

On Tuesday, Belgium issued an international arrest warrant for Mohamed Abrini, described as "armed and dangerous" after he was seen with Salah Abdeslam, the other fugitive suspect, two days before the killings on a highway petrol station en route to Paris.

Abrini and Abdeslam were picked up on the surveillance video of a gasoline station in Ressons.

Abdeslam, who French authorities have suggested could have been linked to a discarded suicide vest found in a southern Paris neighbourhood on Monday, crossed into Belgium the day after the attacks.

Although he was stopped and checked by French authorities, he was allowed to pass as his name had not yet become known.