The US Embassy in Brussels issued a security message on Saturday, urging American citizens to "shelter in place and remain at home" as the Belgian capital was on the maximum alert after security forces found weapons and explosives in a house search.The mission also advised US citizens to avoid large crowds if outdoor travel was necessary. "In this time of elevated threat, the US Embassy in Brussels reminds US citizens to exercise caution in public transportation systems, sporting events, residential areas, business offices, hotels, clubs, restaurants, places of worship, schools, public areas, shopping malls, and other tourist destinations," read the update on the embassy's website. Earlier on Saturday, Belgium put the capital Brussels on maximum security alert shutting the metro and warning people to avoid crowds because of a "serious and imminent" threat of coordinated, multiple attacks by militants.A week after the Paris bombings and shootings carried out by Islamic State militants, of whom one suspect from Brussels is at large and said by police to be highly dangerous, Brussels was placed on the top level "four" in the government's threat scale after a meeting of top ministers, police and security services.Soldiers were on guard in parts of Brussels, including at the institutions of the European Union headquartered in the city. Brussels is also home to the headquarters of NATO."The result of relatively precise information pointed to the risk of an attack along the lines of what took take place in Paris," Prime Minister Charles Michel told a news conference on Saturday. The Paris carnage left 130 people dead."We are talking about the threat that several individuals with arms and explosives would launch an attack perhaps in several locations at the same time," Michel said.He declined to elaborate, but said the government would review the situation on Sunday afternoon.The metro system was to remain closed until then, in line with recommendation of the government's crisis center. Major shopping centers and stores center did open on Saturday morning, with soldiers deployed outside shops. However, many began closing their doors from around midday.The crisis center advised the public to avoid places where a lot of people come, such as shopping centers, concerts, sports events or public transport hubs. The city's museums were shut and concert venues cancelled planned evening events.The agency has called on local authorities to cancel large events and postpone soccer matches, as well as stepping up the military and police presence.Foreign Minister Didier Reynders said that 1,000 troops were now available for patrols, double the level of a week earlier.Fugitive suspected militant Salah Abdeslam, 26, slipped back home to Brussels from Paris shortly after the attacks, in which his elder brother Brahim blew himself up at a cafe.Fears of the risk Salah Abdeslam still poses prompted the cancellation last week of an international friendly soccer match in Brussels against Spain. The crisis center said weekend games in Belgium's two professional divisions should now be postponed, but most outside Brussels appeared set to go ahead.