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President Barack Obama attempted to unite Americans on Saturday, assuring the nation that an attack on police earlier in the week was the work of a "lone wolf" gunman.

Speaking at a NATO meeting in Poland's capital Warsaw, Obama addressed recent racially-motivated violence which has rocked the US after police killed two black men and protests erupted across the country.

Five police officers were later shot dead in Dallas escalating tensions.

Obama described the former army veteran responsible for the killings - 25-year-old Micah Johnson, who was killed by police - as a "demented individual" who did not represent African-Americans.

The Black Lives Matter activist group which have led protests over police brutality has demanded an end to the violence - not an escalation.

As thousands marched in US cities overnight, there were nasty scenes in Phoenix, Arizona, where police used pepper spray to disperse stone-throwing protesters. And in Rochester, New York, 74 people were arrested over a sit-in protest.

Cities expecting new gatherings Saturday included Seattle, Indianapolis and Philadelphia - where organisers called for a "Weekend of Rage."

Obama is due to visit Dallas early next week to smooth tensions and pay his respects to the officers, and is being backed by US presidential hopeful Hillary Clinton.

"White Americans need to do a better job of listening when African Americans talk about the barriers they face,"  Clinton tweeted.

Republican Newt Gingrich appeared to agree: "It is more dangerous to be black in America," Newt Gingrich, a Republican former House speaker said in a Facebook Live interview.

"Sometimes it's difficult for whites to appreciate how real that is. It's an everyday danger."

Republican Florida Senator Marco Rubio had a similar message. "Those of us who are not African American will never fully understand the experience of being black in America," he said.

Republican Donald Trump put out a video statement - seated presidential-style behind a large desk - in which he acknowledged the Louisiana and Minnesota shootings and spoke of "how much work we have to do in order to make every American feel that their safety is protected".

On edge

The Dallas ambush marked the single biggest loss of life for law enforcement in the United States since the September 11, 2001 terror attacks.

Police were set further on edge as it emerged that several officers had been targeted across the country by individuals apparently angered at the recent fatal shootings.

In Bristol, Tennessee, a man opened fire Thursday on a hotel, killing a woman, wounding several others and grazing a police officer with a bullet.

The Tennessee Bureau of Investigation said the gunman "may have targeted individuals and officers after being troubled by recent incidents involving African-Americans and law enforcement officers."

And in Racine, Wisconsin, police said a 43-year-old man was arrested over threatening social media posts that read: "I encourage every Black man in America to strap up... I encourage every white officer to kiss there (sic) love ones goodbye."

Described to police as a "loner," the Dallas gunman served as a US Army reservist for six years, including a tour of duty in Afghanistan.

Although the White House has ruled out any link between Johnson and known "terrorist organizations," his Facebook page ties him to radical black movements listed as hate groups.

He told negotiators before he died that he wanted to kill white cops.

Police found bomb-making materials and a weapons cache at his home and were scouring his journal and social media posts to understand what drove him to mass murder.

Agencies contributed to this story