US protesters welcomed new charges brought Wednesday against Minneapolis officers in the killing of African American man George Floyd -- but thousands still marched in cities across the country for a ninth straight night, chanting against racism and police brutality.

With a key demand met, demonstrators nevertheless staged large and mainly peaceful rallies calling for deeper change in cities from New York to Los Angeles, hours after the new indictments were announced.

Pressure on President Donald Trump mounted as his former Pentagon chief Jim Mattis launched a searing broadside, accusing the Republican leader of trying to "divide" America during the unrest.

In Minnesota, prosecutors had initially charged 44-year-old Derek Chauvin -- the white officer filmed kneeling on Floyd's neck for nearly nine minutes -- with third-degree murder.

But they said Wednesday they were upgrading the charge, roughly akin to manslaughter, to second-degree murder, which does not involve premeditation but carries stiffer penalties.

Chauvin's three colleagues at the scene of Floyd's May 25 arrest for allegedly seeking to buy cigarettes with a counterfeit bill are accused of being complicit in the killing.

Tou Thao, 34, J. Alexander Kueng, 26, and Thomas Lane, 37, were charged with aiding and abetting second-degree murder, and taken into custody.

The arrest of all four officers has been a focus for tens of thousands of protesters who have marched the streets of dozens of US cities, often defying curfews to condemn police brutality and demand racial justice.

Former president Barack Obama applauded the "change in mindset" he sees among Americans demanding racial justice.

He urged the nation to "take the momentum that has been created as a society, as a country, and say 'Let's use this' to finally have an impact."