The Belgian crisis center said on Sunday in a tweet that Brussels remained on a level-four alert, which denotes a "serious and imminent" threat of an attack.

The rest of the country would be at level three, it said, which indicates a "possible and probable" threat.

A further review of the security alert level by ministers and security officials was set for later in the day.

Bernard Clerfayt, the mayor of the Brussels district of Schaerbeek, was quoted by broadcaster RTBF as saying there were "two terrorists" in the Brussels area ready to carry out violence.

Armed patrols

Brussels was locked down on Saturday for a first day under the alert. Authorities closed the city's underground transport system and public buildings, and warned people to keep away from crowded areas in the face of the threat of coordinated attacks by armed militants. Police and soldiers patrolled the streets of the city. Museums, cinemas and shopping centers were also closed. Clubs and venues also canceled events.

Officials fear that jihadis could be planning attacks similar to those in Paris on November 13, which left 130 people dead.

Justice Minister Koen Geens told broadcaster VRT the metro system was likely to resume on Monday. "We will guard the metro stations... We are not going to paralyse Brussels economically, nor the country. We are not led by panic and fear, but we have needed time to reorganise everything," Geens stated.

Suspects at large

Belgian Interior Minister Jan Jambon was quoted by the Belga news agency as saying that security forces were looking for several suspects linked to the Paris attacks.

"We are following the situation minute by minute. There is no reason to hide that. There is a real threat, but we are doing everything possible day and night to face up to this situation," Jambon was cited as telling Flemish television.

Belgium has come to be a focus of investigations into the Paris attacks after it emerged that two of the suicide bombers involved had lived in the country.

tj,das/sms (Reuters, AFP)