US President Barack Obama reacts to the horror of an unarmed man in Minnesota shot by the police. Obama who is in Warsaw for the NATO summit, begged the nation to confront the racial disparities in law enforcement while acknowledging the dangers that officers face.

"When incidents like this occur, there's a big chunk of our citizenry that feels as if, because of the color of their skin, they are not being treated the same, and that hurts, and that should trouble all of us," Obama said in a statement after arriving in Warsaw.

"This is not just a black issue, not just a Hispanic issue. This is an American issue that we all should care about."

He also said he felt compelled to follow up a Facebook message with a personal statement about the killings, though he said he could not comment directly on them. "But what I can say is that all of us, as Americans, should be troubled by these shootings, because these are not isolated incidents," he said. "They're symptomatic of a broader set of racial disparities that exist in our criminal justice system."

The president cited the nation's tortured racial history and current statistics on unequal treatment of the races. He added, "Maybe in my children's lifetimes, all the vestiges of that past will have been cured."

Four police officers have been killed by two snipers who targeted them during a protest at Dallas in Texas, United States over two recent fatal cop shootings of black men.

The snipers shot at 11 police officers, four of whom succumbed to their injuries, Dallas police chief David O Brown said. 

The snipers shot from "elevated positions" during the protest. In addition to the four dead, two are in surgery and four are in critical condition, according to a Dallas Police statement.

Brown said three suspects were custody and officers were continuing to negotiate with one suspect, believed to be one of the shooters, who was cornered on the second floor of a garage.

He added that he did not have a "complete comfort level" that all suspects had been apprehended.

Officers were continuing to exchange gunfire with the cornered suspect.