Belgium widened its search on Sunday for armed Islamist extremists whose presence has put Brussels on maximum alert, with officials saying more than one militant was at large in the city.

The alert has closed the capital's metro system, although a senior minister said it was likely to reopen on Monday.

Belgium has been at the heart of investigations into the Paris attacks on November 13 that left 130 people dead after links emerged to Brussels, and the poor district of Molenbeek in particular.

Two of the Paris suicide bombers, Brahim Abdeslam and Bilal Hadfi, had been living in Belgium. Fugitive suspected militant Salah Abdeslam, Brahim's 26 year-old brother, slipped back home to Brussels from Paris shortly after the attacks.

Asked whether Brussels' maximum threat level since Saturday related to Salah Abdeslam alone, Interior Minister Jan Jambon told broadcaster VRT "unfortunately not".

"It is a threat that goes beyond just that one person," he said. "We're looking at more things, that's why we've put in place such a concentration of resources."

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.

Mohamed Abdeslam, the brother of Brahim and Salah, urged Salah in an interview on RTBF television to give himself up, adding that he believed Salah was still alive because he had had a last-minute change of heart while in Paris.

Belgium's crisis centre, a state body that advises the government on security, said on Sunday the alert status for Brussels remained at its highest level of four, meaning a "serious and imminent" threat of an attack.

ALERT STATUS

Intelligence, police and judicial officials would review the alert status during the course of the day. The national security council, including top ministers, was expected to convene on Sunday afternoon to determine what measures to take or retain.

Justice Minister Koen Geens told 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 said.

Prime Minister Charles Michel has advised the public to be alert rather than panic-stricken, but also said Brussels risked Paris-style coordinated attacks.

Belgium has urged the public to avoid crowds in the capital, and also closed museums, cinemas and shopping centres. Clubs and venues have cancelled events.

Brussels Chief Rabbi Albert Gigi told Israel's Army Radio on Sunday that the city's synagogues were shut over the weekend for the first time since World War Two.

Soldiers are on guard in parts of Brussels, a city of 1.2 million people and home to institutions of the European Union and the headquarters of NATO.

That said, Brussels on Sunday morning resembled most other Sundays, with the normal limited number of shops, such bakeries and small supermarkets open, and many churches in the largely Catholic country still holding services.

However, larger markets were shut.

The weekend's measures go far beyond those taken the last time Brussels was put on level four alert, for about a month at the end of 2007 and the start of 2008, when authorities intercepted a plot to free convicted Tunisian Nizar Trabelsi from a Belgian jail.

Then the city closed the downtown Christmas market early and cancelled its New Year fireworks display. - Reuters