From the U.S. Embassy in Belgium:
The U.S. Embassy in Brussels informs U.S. citizens that Belgium has raised its threat level for Brussels from level 3 (high) to level 4 (imminent attack). For the rest of Belgium, the threat level remains at level 3. The U.S. Embassy urges U.S. citizens to shelter in place and remain at home. If you must go out, avoid large crowds. The Embassy will provide further information as received and is available.