Speaking in the run-up to the NATO summit in Warsaw, Poland, Obama expressed his condolences with the families of the two victims of recent high-profile police shootings, one in Minnesota and the other in Louisiana.

"Many Americans feel it's because they're not being treated the same and that hurts," he said.

Alton Sterling was shot early on Tuesday in Baton Rouge, Louisiana, as he struggled with two white police officers outside a convenience store. A day later Philando Castile, another black man, was shot and killed by police in Minnesota.

Thousands of Americans staged protests in Washington, New York, Los Angeles, and several other cities on Thursday to voice their outrage over the two shootings.

Police said one transit officer had been fatally shot after gunfire erupted at a protest in central Dallas. Three others were injured.

Earlier, US media reports said two police officers had also been shot at the same Dallas march. There was no immediate information about the officers' condition.

Footage from the scene showed a heavy police presence, with officers taking cover behind their cars and emergency workers keeping protesters away.

'Not just a black issue'

"We can do better than this," Obama told reporters in Warsaw, adding that to be concerned about such things was "not to be against law enforcement." He said that reform of the police has been "too slow" as he cited statistics showing African-Americans were being shot by police or arrested at more than twice the rate of white Americans.

Obama said "all fair-minded Americans" should be concerned about the problem of frequent police shootings of black people. "But it's not just a black or Hispanic issue," the president added.

Obama spoke at a hotel in Warsaw just after arriving in Poland for a NATO summit. He largely echoed comments he made earlier in the day in a Facebook post as the two deaths were increasingly capturing the country's attention.

He had said it was clear the slayings were not isolated incidents. [These] are "symptomatic of the broader challenges within our criminal justice system, the racial disparities that appear across the system year after year, and the resulting lack of trust that exists between law enforcement and too many of the communities they serve," Obama wrote in a Facebook post.

The Justice Department launched a civil rights investigation on Wednesday into Sterling's shooting.

jbh,nm/jm (AP, Reuters)