Belgium maintained the highest security alert for Brussels Monday with the metro system and schools to re-open only Wednesday after authorities said there was an imminent threat of a Paris-style attack in the Belgian capital.

The alert level for Brussels would remain at the highest level four while in the rest of the country it would stay at three.

"We are still confronted with the threat we were facing yesterday," Belgian Prime Minister Charles Michel told a news conference. He said potential targets remained shopping malls, shopping streets and public transport.

"We want progressively to return to normality... Schools in Brussels will open on Wednesday and the metro on Wednesday as well. For the metro it could be in a progressive way," he said.