WASHINGTON (AFP) - US Attorney-General William Barr said on Saturday (May 30) that far-left extremists and anarchists were behind the violent protests against policy brutality in more than a dozen cities, and warned federal law enforcement could take action against them.

While local leaders in hard-hit Minneapolis, the epicentre of the protests, said organised white supremacist groups and drug gangs could have had a significant role in setting fires and looting, Barr echoed President Donald Trump's claim that the left was responsible.

"Unfortunately, with the rioting that is occurring in many of our cities, around the country, the voices of peaceful protest have been hijacked by violent radical elements," he said in a brief televised statement.

"In many places, it appears the violence is planned, organized and driven by anarchic and far-left extremist groups using Antifa-like tactics, many of whom travel from outside the state to promote the violence."

Antifa is a loose-knit group of leftist militants saying they are fighting fascism.

The protests in Minneapolis and a dozen more major cities Friday were sparked by the killing of African American George Floyd in the custody of Minneapolis police on Monday.

The protests in many places turned violent and have included several nights of fires and looting, with the Minnesota city the hardest hit.

Earlier, Trump tweeted likewise that protesters were "Antifa and the Radical Left."

"Don't lay the blame on others!" he said.

Trump also said that hundreds of protesters who chanted outside the White House late on Friday were "professionally managed" and "had little to do with the memory of George Floyd."

"They were just there to cause trouble," he tweeted.

Earlier, Minnesota Governor Tim Walz said intelligence pointed to groups of anarchists, rival narcotics gangs, and white supremacists deliberately fomenting violence and chaos, many of them from outside the city.

Barr said that it was still the responsibility firstly of state and local officials to deal with the violence.

However, he said, "We must have law and order on our streets, and in our communities."

The Justice Department, the FBI and other federal law enforcement agencies are providing support to local authorities, he added.

Barr warned that the Justice Department could weigh in by enforcing federal laws against crossing state lines to participate in the violence.

"We will enforce those laws," he said.