Belgium raised the alert status for its capital Brussels to the highest level on Saturday, warning the public to avoid crowds because of a "serious and imminent" threat of an attack, a government official said.
Brussels was placed on the top level "four" in the government's threat scale after a meeting of top ministers, police and security services.
"The advice for the population is to avoid places where a lot of people come together like shopping centers, concerts, events or public transport stations wherever possible" a spokesman for the government's crisis center said.
The level for the whole country was raised a week ago following the Paris attacks to level three out of four, implying a "possible or probable" threat. Previously, only certain sites, such as the U.S. embassy, were at level three.
Belgium, and its capital in particular, have been at the center of investigations into the Paris attacks after it emerged that two of the suicide bombers had been living in the country. Three people detained in Brussels are facing terrorism charges.
The brother of one of the suicide bombers, who was also living in Brussels, is still on the run. Salah Abdeslam was briefly pulled over by French police near the Belgian border early last Saturday morning along with two of those in custody.
Some Belgian media speculated that the heightened state of alert related to the continued search for him.
The crisis center spokesman declined to say what had led to the status change because investigations were proceeding.
"We cannot give more information... The work of federal prosecutors is still going on," he said, adding the government was assessing what extra security measures to take. Soldiers are already on guard in certain parts of Brussels.
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 canceled.
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.