WASHINGTON (REUTERS) - Democrats said Mr Donald Trump should not visit the Wisconsin city where protests erupted last week after a black man was shot in the back by a white police officer, while the Republican president said "strength" was the only way to deal with unrest.

The Aug 22 shooting of Jacob Blake in front of three of his children turned Kenosha, a mostly white city south of Milwaukee, into the latest flashpoint in a summer of US demonstrations against police brutality and racism, ahead of Trump's November re-election bid.

Mr Trump has taken a hard stand against racial protests in the country and the White House said he will visit the Midwestern city on Tuesday, raising concerns among Democrats that this may worsen the strife.

"They centred an entire convention around creating more animosity and creating more division around what's going on in Kenosha," Wisconsin Lieutenant-Governor Mandela Barnes told CNN, referring to last week's Republican National Convention.

"So I don't know how, given any of the previous statements that the president made, that he intends to come here to be helpful, and we absolutely don't need that right now," he added.

Critics accuse Trump, who faces Democratic former vice-president Joe Biden in the Nov 3 presidential election, of seeking to exacerbate violence with incendiary rhetoric, while the president has repeatedly called in tweets for "law and order."

In a statement on Sunday, Biden accused Trump of "recklessly encouraging violence."

"He may believe tweeting about law and order makes him strong - but his failure to call on his supporters to stop seeking conflict shows just how weak he is. He may think that war in our streets is good for his reelection chances, but that is not presidential leadership - or even basic human compassion."

Republicans accuse Democratic mayors and state governors of losing control of cities rocked by demonstrations that have seen outbreaks of violence, arson and vandalism.

US Representative Karen Bass, who chairs the Congressional Black Caucus, said Trump's trip to Kenosha would only increase tensions.

"His visit has one purpose and one purpose only, and that is to agitate things," the Democrat told CNN's State Of The Union programme.

"We're 66 days from an election and I think it's a tragedy that we have a president that is doing everything he can to fan the flames."

Anger over Blake's shooting sparked three nights of civil unrest in Kenosha, including clashes between anti-racism protesters and armed militia members.

On Tuesday night a white teenager with a semi-automatic rifle shot three demonstrators, killing two of them.

The 17-year-old suspect, Kyle Rittenhouse, is being held without bond after being charged with six criminal counts, including first-degree homicide and attempted homicide in connection with the incident, which was captured in several videos recorded by witnesses.

His lawyers say he acted in self-defence after travelling to Kenosha from his home 50km away in Antioch, Illinois, to help protect businesses during the unrest, and they called his prosecution "a reactionary rush to appease the divisive, destructive forces currently roiling this country."

Rittenhouse has also received support from some right wing commentators who have hailed the former YMCA lifeguard as a hero who wanted to defend his community and support law enforcement.

Another flashpoint in the tensions between demonstrators and counterprotesters has been Portland, Oregon, where one person was shot and killed late on Saturday after three months of nightly protests that began with the May 25 death of George Floyd in Minneapolis police custody.

The shooting in Portland came after a caravan of Trump supporters drove into the downtown area where there were confrontations with protesters, according to videos posted on social media. Police urged the public to let detectives do their work before drawing conclusions about what took place.

Overnight, Trump sent several tweets and retweets criticising Portland's Democratic mayor, Ted Wheeler, and again urging him to request help from federal law enforcement.

"The people of Portland, like all other cities & parts of our great Country, want Law & Order," Trump wrote on Twitter on Sunday. "The only way you will stop the violence in the high crime Democrat run cities is through strength!"

Biden has defended peaceful protesters, saying justice must be done, but he has repeatedly called for an end to violence. In his statement on Sunday, he called the Portland violence "unacceptable."

Asked on CBS' Face The Nation whether Trump's social media posts were heightening tensions, acting Homeland Security secretary Chad Wolf said "absolutely not."

Pressed on whether local law enforcement should crackdown on violence by pro-Trump groups as well as by anti-racism protesters, Wolf said he wanted them to "address any violent activity."

White House Chief of Staff Mark Meadows echoed that the federal government stood ready to provide law enforcement support to any governor, Republican or Democrat, who needed it.

"We are willing to come in; we are willing to provide additional assets as we did in Kenosha," Meadows told NBC's Meet The Press programme.