At least one sniper killed five Dallas police officers and wounded seven others in a racially charged attack that ended when police used a robot carrying a bomb to kill him, the city's police chief said on Friday.

The Thursday night killings, at the end of a protest over this week's pair of fatal shootings of black men by police in Louisiana and Minnesota.

A long string of killings of black men by police in cities including Ferguson, Missouri, New York, Baltimore and Chicago have given rise to the Black Lives Matter movement against excessive police force. The killings have spurred almost two years of largely peaceful street protests.

Thursday's shooting sent protesters running in panic while swarms of police found themselves under attack by what they believed to be multiple gunmen using high-powered rifles at ground level and on rooftops.

The suspect was identified as Micah X. Johnson, who was a member of the US Army Reserve, a U.S. government source told Reuters.

During lengthy negotiations with police, the gunman said "the end is coming," according to Dallas Police Chief David Brown.

"The suspect said he was upset about Black Lives Matter," said Brown, who is black. "He said he was upset about the recent police shootings. The suspect said he was upset at white people. The suspect stated that he wanted to kill white people, especially white officers."

Brown declined to say how many people took place in the attack. "We're going to keep these suspects guessing," he told reporters at City Hall.

Police said they were questioning two occupants of a Mercedes they had pulled over after seeing a man throwing a camouflage bag inside the back of the vehicle, which then sped off on a downtown street. A woman was also taken into custody near the garage where the standoff took place.

Quinyetta McMillon, who had a child with Alton Sterling, the black man slain by police in Baton Rouge, Louisiana, earlier this week, condemned the Dallas attack in a statement.

"Regardless of how angry or upset people may be, resorting to this kind of sickening violence should never happen and simply cannot be tolerated," McMillon said.