Flames engulf US police station as black rights unrest spreads  AFP  

GETTY IMAGES NORTH AMERICA/AFP / SCOTT OLSONA police building went up in flames in Minnesota during protests

Flames ripped through a Minnesota police station and seven protesters were shot in Kentucky as unrest spread across the United States over the deaths of black people during police encounters.

Officers abandoned the building in the city of Minneapolis late on Thursday before demonstrators barged through barriers, breaking windows and chanting slogans. A fire broke out, which soon became an inferno that engulfed the structure.

The protests entered their fourth day on Friday and have spread beyond Minnesota, with protests breaking out in several states across the country, including Denver, Colorado and Phoenix.

In Kentucky, seven people were hit by gunfire at a protest on Thursday over the death of Breonna Taylor -- a black woman who was shot after police entered her home in March, local media reported.

One of those wounded was in a critical condition, according to the Louisville Metro Police Department. It is not yet clear who fired the shots.

Police responded with a Twitter post asking the city to "please choose peace," alongside a video message from a family member of the woman killed. She asked those in the streets to "go home and be safe and be ready to keep fighting."

- 'Thugs' -

Thousands joined the protests in Minnesota, which were triggered by the Monday death of 46-year-old George Floyd after being arrested on suspicion of using a counterfeit banknote.

A video taken by a bystander shows an officer kneeling on his neck as he is pinned to the ground. At one point Floyd is heard saying he cannot breathe.

As unrest spread, President Trump tweeted: "These THUGS are dishonouring the memory of George Floyd and I won't let that happen," in apparent reference to protesters in Minnesota, adding the state's governor has the backing of the military.

GETTY IMAGES NORTH AMERICA/AFP / Stephen MaturenA large crowd gathered outside a police precinct to protest the death of George Floyd after his arrest by police

"Any difficulty and we will assume control but, when the looting starts, the shooting starts."

Twitter flagged the tweet for violating its rules on glorifying violence a few hours after it appeared.

Minnesota's governor Tim Walz earlier called up 500 of the state's National Guard, but after Trump tweeted he defended his decision not to put them on the streets as fire gripped the police station.

"Bricks and mortar are not as important as life," Minneapolis mayor Jacob Frey said at a press conference, adding that officers had been deployed in the city to prevent looting.

He added that the "anger and frustration" on the streets must be understood, but that the looting was unacceptable.

"Our communities cannot and will not tolerate it. These are businesses, these are community institutions that we need," Frey said.

GETTY IMAGES NORTH AMERICA/AFP / Stephen MaturenOfficials have warned violence will not be tolerated

The City of Minneapolis called for people in the third precinct -- where the police building is located -- to leave for their own safety, saying "explosive materials" could be inside and gas lines to the area had been cut.

Outbreaks of violence have gripped the city as tensions rose since Floyd's killing.

On Wednesday, demonstrators clashed with law enforcement, looted stores and set fire to shops and a construction site. They were met with police tear gas and rubber bullets.

One person died of a gunshot wound, and police were reportedly investigating whether he was shot by a store owner.

- 'I have not slept' -

The Justice Department on Thursday promised to carry out a "robust" investigation into Floyd's death, saying they will make the case a top priority.

Democrats have also called for a probe into the deaths of two other black people -- Ahmaud Arbery, who was shot by two white men in the city of Brunswick in Georgia, and Breonna Taylor.

Floyd's family has demanded the officers present when he died face murder charges. All of them have been fired.

"You know, I want an arrest for all four of those officers tonight. A murder conviction for all four of those officers. I want the death penalty," Floyd's brother, Philonise Floyd, told CNN.

"I have not slept in four days, and those officers, they're at home sleeping," he said.

Two African American leaders of national stature, Jesse Jackson and Al Sharpton, arrived in Minneapolis Thursday and urged more protests.

"We told the governor you must call murder a murder," Jackson told an audience at the Greater Friendship Missionary Baptist Church.

500 National Guard troops deployed in Minneapolis

AFP / Kerem YucelProtesters 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

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.

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.