Harrowing footage of George Floyd's death in police custody left US President Donald Trump "very upset" as authorities brace for another night of violent riots sparked by the incident.
Mr Floyd, a 46-year-old restaurant worker, died in the northern city of Minneapolis on Monday while in police custody, sparking angry and violent protests in which one man was shot dead.
A bystander video showed a handcuffed Mr Floyd gasping for breath as a policeman pressed his knee on his neck after detaining him for allegedly using fake money for a purchase.
The White House said President Donald Trump was "very upset" upon seeing "egregious, appalling" video footage of Monday's killing and had demanded his staff see that the investigation was given top priority.
Mr Trump said the death was a "very, very sad event".
The FBI and Justice Department said they were investigating the death, while the city has called in National Guard troops to help keep the peace ahead of more planned protests.
The UN human rights chief has condemned the death of Mr Floyd, demanding that US authorities take "serious action" to stop the killings of unarmed African Americans.
"This is the latest in a long line of killings of unarmed African Americans by US police officers and members of the public," Michelle Bachelet said in a statement.
"I am dismayed to have to add George Floyd's name to that of Breonna Taylor, Eric Garner, Michael Brown and many other unarmed African Americans who have died over the years at the hands of the police," she said.
Demonstrators have clashed with police, looted stores and set fire to shops, with police reacting with tear gas and rubber bullets.
One person was also reported dead Thursday of a gunshot wound, with police investigating whether he was shot by a shop owner in the area hit by the most severe rioting.
After stores were looted and burned overnight Wednesday in Minneapolis, officials in the so-called Twin Cities warned they would not tolerate violence while seeking to assure that investigations into George Floyd's death were underway.
Hundreds of people began marching in Minneapolis in the late afternoon, while in St. Paul, just to the east, police said there was ongoing looting.
"We know there's a lot of anger. We know there's a lot of hurt," said St. Paul Police Chief Todd Axtel.
"But we can't tolerate people using this as an opportunity to commit crimes," he said.
The chief of the Minneapolis police on Thursday apologised to Mr Floyd's family.
Police Chief Medaria Arradondo said his department had contributed to a “deficit of hope” across the Minnesota city even before the death of 46-year-old George Floyd on Monday.
“I am absolutely sorry for the pain, devastation and the trauma that Mr. Floyd’s death has left on his family, his loved ones and our community,” he said
