Police armed with automatic weapons guarded schools in the Belgian capital as they reopened Wednesday for the first time since emergency measures were imposed in the wake of the Paris attacks, while Belgian and French authorities continued a hunt for at least two suspects believed directly linked to the killings.

Brussels' subway system also began reopening after a four-day shutdown, with about half of all stations operational. Despite the easing of restrictions, the city remained under the highest threat alert level due to what authorities have described as a serious and imminent threat of attack. Emergency measures imposed since Saturday had shut down shops, schools and subways.

The threat alert level is to remain in place until at least Monday barring significant developments such as the arrest of suspects linked to the Paris attacks. Meanwhile, the Belgian government has ordered health and emergency services to take precautionary measures to ensure their services aren't 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 Sunday night had been designed to foil an imminent attack in Brussels. Authorities had detained 16 people during the raid, but released all but one of them the following day. No explosives or firearms were seized.

"Indeed, there were indications that there would be attacks on Sunday evening and they did not materialize," Jambon said. Otherwise, he said, "you don't impose terror level 4," the highest possible. The minister refused to elaborate what kind of attacks the government believed had been planned.

The reopening of schools and some subway stations is restoring a sense of normality to Brussels, parts of which have been deserted since the alert was first raised.

At Brussels' 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 visible machine gun. Among the students at the school is Belgium's Princess Eleonore.

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

"I'm concerned, but I think that life must go on," said Dimitri De Cra Yencour, a father of four. "Even if something happens in Paris or in Brussels, they have to go back to school," he said, adding he had instructed his children to be extra attentive and to tell their teachers if they see anything unusual.

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 in shootings and suicide bombings in the French capital. Belgium on Tuesday 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 gas station on the way 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 neighborhood 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.

One of Abdeslam's brothers, Brahim, was the suicide bomber who blew himself up outside a Paris cafe during the attacks. His other brother, Mohamed, appealed on French media Wednesday for his fugitive sibling to surrender to police.

Speaking on RTL radio, Mohamed Abdeslam said he shares the pain of victims' families and wishes he and his family could have done something to prevent the Nov. 13 bloodshed.

"Let him turn himself in for his parents, for justice, for the families of victims, so that we can find out what happened," Mohamed Abdeslam said.

He said his brothers had shown no signs of radicalization. Mohamed Abdeslam said he saw them a few days before they left their Brussels suburb for Paris, but had no idea what they were plotting, and hasn't heard from Salah since.

Emergency measures have also been taken in France while the manhunt continues.

U.N. climate talks open in Paris on Monday, with about 140 world leaders expected to attend, including President Barak Obama. French Interior Minister Bernard Cazeneuve said Wednesday that tight security would be imposed for the conference, with road traffic restrictions, border controls and additional police and troops deployed.

Cazeneuve said 120,000 police, gendarmes and soldiers are deployed across France to ensure the country's security.