Several economists have projected that Donald Trump's economic agenda would slow economic growth and possibly cause a recession (AP)

Donald Trump has insisted drugs played "a very, very big factor" in violent protests that erupted in North Carolina overnight following the police shooting of a black man.

The Republican presidential candidate warned African-American protesters that their outrage was creating suffering in their own communities.

It was another day of mixed messages on a delicate issue from Mr Trump, who was eager to blunt criticism that his campaign inspires racism in the midst of what he called "a national crisis".

The National Guard was activated to calm fierce protests that followed police shootings of black men in North Carolina and Oklahoma.

Mr Trump has sought to express empathy, but his words could rankle some in the African-American community.

"The people who will suffer the most as a result of these riots are law-abiding African-American residents who live in these communities where the crime is so rampant," Mr Trump declared at an energy conference in Pittsburgh.

He added: "Drugs are a very, very big factor in what you're watching on television at night."

Democrat Hillary Clinton did not address escalating racial tensions on Thursday as she prepared for her first debate-stage meeting with Mr Trump.

She took a swipe at her opponent, albeit in a humorous way, in an interview on comic Zach Galifianakis' internet show Between Two Ferns.

The comedian asked her what Mr Trump might wear to Monday's debate.

"I assume he'll wear that red power tie," Mrs Clinton said. Galifianakis responded: "Or maybe like a white power tie."

"That's even more appropriate," Mrs Clinton said.

Both candidates are working to navigate the politics of race with Election Day less than seven weeks away and early voting about to begin in some states.

Mr Trump, in particular, has struggled to balance a message that appeals to his white, working-class base with one that improves his standing with minorities and educated whites who may worry about racial undertones in his candidacy.

He was slow to disavow former KKK leader David Duke earlier in the year and has repeatedly promoted tweets by white supremacists during his White House bid.

The Republican nominee admitted for the first time publicly last week that President Barack Obama was born in the United States. And as recently as last week, Mr Trump's eldest son tweeted a meme commonly used by white nationalists.

On Thursday, Mr Trump tried at times to project a softer message, calling for a nation united in "the spirit of togetherness".

"We all have to walk a mile in someone else's shoes, see things through their eyes, and then get to work fixing our very wounded country."

The message was complicated by his own suggestion that protesters outraged by the police shootings of black men were under the influence of drugs. Earlier in the day, he also called for Chicago to adopt "stop and frisk" policing tactics that have been condemned as racial profiling.

At the same time in neighbouring Ohio, Mr Trump's Mahoning County chair Kathy Miller, a volunteer, came under fire after telling the Guardian newspaper: "I don't think there was any racism until Obama got elected."

The Trump campaign accepted her resignation after what a spokesman called "inappropriate" comments.

Mrs Clinton has faced criticism of her own for saying half of Mr Trump's supporters belong in a "basket of deplorables" because they are racist, sexist, homophobic or xenophobic.

The Democratic nominee has also made curbing gun violence and police brutality a central part of her candidacy.

She said on Wednesday that shootings in Oklahoma and North Carolina added two more names "to a long list of African-Americans killed by police officers. It's unbearable and it needs to become intolerable".

Mrs Clinton has campaigned alongside a group of black women called the Mothers Of The Movement, who advocated for more accountability and transparency by law enforcement. The group includes the mothers of Eric Garner, Trayvon Martin and Michael Brown, black victims of high-profile killings.

Mr Trump said new leadership is required to address the situation.

"This is a national crisis," he said without mentioning the black men shot by police in recent days.

He said that "it's the job of the next president of the United States to work with our governors and mayors to address this crisis and save African-American lives".

AP