Several active-duty military police units have been told to be ready to deploy within four hours, if called upon to enter the US state of Minneapolis.
It comes as rioting in the city caused by the death of an unarmed African-American man at the hands of a police officer continued for a fourth consecutive night.
Protests have spread to at least 20 US cities with one person fatally shot in Detroit. Police say a 19-year-old man was killed by shots fired from a SUV, near the city's Greektown entertainment district.
