Dallas shooting: Gunman told police negotiator he 'wanted to kill white people'

One of the suspects behind the sniper killing of five police officers in Dallas told a negotiator he "wanted to kill white people", especially white police officers, the city's chief police officer says.

The suspect also said he was not affiliated with any other groups and acted alone, Chief David Brown told reporters during a press conference.

The suspect, who had engaged in a shoot-out for 45 minutes with police during a Black Lives Matter Protest in Dallas, was killed by a police bomb, while three other suspects were detained.

"The suspect said he was upset about Black Lives Matter," Chief Brown said.

"He said he was upset about the recent police shootings.

"The suspect said he was upset at white people ... he wanted to kill white people, especially police officers."

Five police officers were shot and killed and seven more injured when snipers in an elevated position opened fire in the city's downtown area.

The peaceful rally was one of a number across the US in response to two fatal police shootings of black men, in Minnesota and Louisiana, this week.

One female suspect was taken into custody after being seen near a multi-storey car park; two more suspects were stopped after driving away in a Mercedes with camouflage bags.

Some of the officers targeted were traffic police and were not wearing Kevlar body armour as they wanted to avoid presenting an aggressive posture at what was supposed to be a peaceful protest.

Some officers were "ambushed" and shot, some in the back, Chief Brown said.