Brussels in lockdown

Many in Brussels continued to go outside despite the threats [AP Photo/Virginia Mayo]

Belgium's state of emergency has entered a second day amid concerns that a "Paris-style" attack involving explosives is imminent in Brussels, which is on lockdown with the army on the streets.

Shops in the Belgian capital are closed, the city's metro system has been shut down, and residents have been urged to say at home in the face of a "serious and imminent threat".

Warnings on movement remained in place on Sunday as authorities continued their efforts to track down one of the men believed to be involved in the attack on the French capital last week that killed 130 people.

Salah Abdesalam, a Belgian national, is suspected to be hiding in the Brussels area and carrying what might be an explosive device.

Charles Michel, Belgium's prime minister, moved to increase the state of alert in Brussels to the highest level early on Saturday in response to the threat.

Metro services and major events have since been suspended as hundreds of police officers and soldiers fanned out across the city.

Residents have been asked to stay indoors and avoid crowded areas amid the alert, which is in force only in Brussels.

Emergency phone lines have been set up to report suspicious activity, as well as sightings of Abdesalam.

Ongoing threat

Belgium has been at the forefront of efforts to track down those involved in helping execute the November 13 attacks in Paris, which targeted a concert hall, football stadium, cafe and shopping mall.

On Thursday, the country's security services launched raids in the Molenbeek and Jette neighbourhoods of Brussels and made further arrests on Saturday.

The attacks were blamed on the Islamic State of Iraq and the Levant (ISIL) group, and several of the attackers are believed to have received training in Syria. 

The apparent ringleader, Abdelhamid Abaaoud, was a Belgian national who fought with ISIL and is suspected of helping to inspire or direct previous attacks targeting France.

Since the start of October, ISIL has launched a number of attacks against targets outside the territory it primarily operates in, including in France, Lebanon, and Turkey.

The group has also claimed credit for bringing down on October 31 a Russian airliner in Egypt's Sinai Peninsula by smuggling a bomb on board.