LIVE: Explosions kill dozens in Brussels terror attacks

BRUSSELS, Belgium -- The U.S. Embassy in Brussels is recommending that Americans in Belgium stay where they are and avoid public transportation.

PHOTOS: Aftermath of terror attacks in Brussels

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Solidarity messages are written in chalk outside the stock exchange in Brussels on Tuesday, March 22, 2016.

Geert Vanden Wijngaert/AP

The embassy noted Tuesday that with the threat rating in Brussels at its highest alert, attacks can take place with little or no notice. It urged U.S. citizens to monitor media reports, follow instructions from the authorities, and "take the appropriate steps to bolster your personal security."