Five hundred National Guard soldiers and airmen have been deployed in the northern US cities of Minnesota and St. Paul after three nights of violent protests over the police killing of a black man, the force said Friday.

"Our troops are trained to protect life, preserve property and ensure people's right to peacefully demonstrate," said Major General Jon Jensen of the Minnesota National Guard.

The deployment began overnight as rioters looted and burned dozens of buildings across the Twin Cities amid anger of police brutality against African Americans.

Protesters hold up their fists as flames rise behind them in Minneapolis, Minnesota, during a protest over the death of African American George Floyd in police custody Protesters hold up their fists as flames rise behind them in Minneapolis, Minnesota, during a protest over the death of African American George Floyd in police custody Photo: AFP / Kerem Yucel

The guardsmen and women were processed into duty overnight and will be on rotating missions, supporting local and state police in peacekeeping jobs from Friday morning, the guard said.

"The National Guard will provide support to civil authorities as long as directed in order to ensure the safety of people and property," they said.