President Donald Trump says federal law enforcement will be sent to Kenosha, Wisconsin, following unrest sparked by the police shooting of a black man.
Jacob Blake, 29, was shot and injured by police on Sunday as he leaned into his car as his children watched.
Protests have been ongoing for several nights, and on Tuesday, two people were shot dead and another was injured. A teenager has been charged with murder.
Mr Blake is recovering in hospital and is conscious, his family say.
But his lawyers say it will take “a miracle” for him to walk again.
The protests over Mr Blake’s shooting have at times turned violent, causing damage to property.
Tuesday night’s incident was believed to have involved protesters against Mr Blake’s shooting and armed men guarding a petrol station.
Hours later, police in neighbouring Illinois said they arrested a 17-year-old on suspicion of first-degree murder.
Announcing his move in a tweet on Wednesday, Mr Trump said: “We will NOT stand for looting, arson, violence, and lawlessness on American streets. My team just got off the phone with Governor Evers who agreed to accept federal assistance (Portland should do the same!).”
Shortly following Mr Trump’s tweet, Wisconsin Governor Tony Evers said he had authorised 500 National Guard troops to support law enforcement efforts.
Kenosha Mayor John Antaramian told reporters on Wednesday: “I am here today basically to, number one, inform everyone that we are not planning on letting this violence continue.
“Number two, we are going to work together to resolve the racial issues in our community, and we are going to make that work.”