Dallas police exchanged gunfire early Friday with a suspect in a deadly shooting that has killed four officers who warned negotiators there were “bombs all over the place” in downtown Texas, officials said.

“The suspect that we are negotiating with that has exchanged gunfire with us over the last 45 minutes has told our negotiators that the end is coming, and he is going to hurt and kill more of us, meaning law enforcement. And that there are bombs all over the place in this garage and in downtown,” Police Chief David Brown told reporters.

“So we are being very careful in our tactics so we don’t injure our citizens in Dallas as we negotiate further.

Four police officers were killed, at least 10 were wounded and a civilian was also hurt late Thursday when two snipers opened fire during a protest in Dallas against police racial bias, authorities said.

City Police Chief David Brown said two gunmen shot at "police officers from elevated positions during the protest/rally", sparking an intense search for at least two suspects, adding that they had threatened to plant a bomb.

The Dallas Police Department tweeted a photograph of a man dressed in a camouflage shirt with an assault rifle slung over his shoulder.

"This is one of our suspects. Please help us find him!" the police department tweeted.

The suspects "have threatened to plant a bomb in the downtown area," Brown told reporters.

The protests were part of demonstrations taking part in several cities against the fatal police shootings of African American men in Minnesota and Louisiana this week.

Brown initially said two of the officers were in surgery and three were in critical condition. The condition of the other officers was unclear.