Belgium shuts down its metro and raises the terror alert to its highest level because of a 'threat of an attack ... in the capital'

Belgium raised its terror alert to the highest level in the capital Brussels on Saturday, also shutting down the metro and warning the pubic to avoid crowds.

Belgian Prime Minister Charles Michel on Saturday said the decision to raise the terror threat level in the capital Brussels to the highest possible mark was linked to a threat of imminent attacks.

"This was due to a threat of an attack by individuals with explosives and weapons at several locations in the capital," Michel told a news briefing in Brussels.

"Following our latest evaluation ... the centre has raised its terror alert to level 4, signifying a very serious threat, for the Brussels region," a statement on the crisis centre's website said, as Europe tightens security after Islamic State militants killed 130 people in the French capital.

The centre also recommended the underground rail network stay closed until Sunday and urged citizens to avoid crowded areas such as concerts and transport hubs in Belgium's capital, which is also home to the European Union and NATO headquarters.

The officials declined to add further details until later on Saturday "in order to allow ongoing judicial investigations to follow their course," the statement said.

Belgian investigators a few hours earlier charged a third unnamed suspect, arrested the day before, with involvement in terrorism over the Paris attacks.

This was in addition to Hamza Attou and Mohammed Amri, who were charged earlier this week with aiding fugitive Paris attacks suspect Salah Abdeslam on the night of the attacks.

Reuters//Francois LenoirPolice near the Brussels metro, which closed Saturday.

Brussels resident Abdeslam, whose brother Brahim blew himself up at a Paris bar, is believed to be in Belgium and is the subject of a huge manhunt.

The fugitive is closely linked with the alleged Paris attack ringleader Abdelhamid Abaaoud, who was killed in a police raid on Wednesday.

Both grew up in the poor Molenbeek district of Brussels, described as an extremist hotbed, before going to join the Islamic State group in Syria.

The last time any part of the country was put on maximum alert was in May 2014 when a gunman shot dead four people at the Jewish Museum in Brussels. At that time, Jewish schools, synagogues and other institutions were put on level four.

The capital as a whole was last at the level four 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. Brussels' traditional New Year fireworks display was cancelled.

Trabelsi was sentenced in Belgium in 2003 to 10 years for attempting to blow up a Belgian military base that houses U.S. soldiers. He was extradited to the United States in 2013.

The government's four-level alert system has been in place since 2006.