A police officer has shot and killed an unarmed 19-year-old black youth who allegedly assaulted him in the US city of Madison, Wisconsin, the city's police chief said.

The incident on Friday evening started off protests in the city, and local media reported a heavy police presence in the area where the shooting occurred.

Police chief Mike Koval told WKOW television the police officer was responding to a report of a battery and had forced his way into an apartment after hearing sounds of a disturbance inside.

"Once inside the home, the subject involved in the incident - the same one who had been allegedly out in traffic and had battered someone - this same subject then assaulted my officer, and in the context of mutual combat in that sense, the officer did draw his revolver and subsequently shot the subject," Koval said.

Koval said the officer then administered CPR, and the wounded youth was taken to a hospital where he died.

He said an initial search found no gun or other weapon at the scene.

The shooting was under investigation by the city's Department of Criminal Investigations, Koval said.

It was the latest in a string of police shootings of young blacks that have set racial tensions on edge in the United States, igniting a nationwide debate over police tactics in minority communities.

A large crowd of chanting protesters formed Friday night at the scene of shooting.

The shooting comes just days after the US Justice Department said it would not prosecute the white policeman who shot an unarmed black teenager in Ferguson, Missouri, whose death sparked riots and outrage.

But the report did find that the St Louis suburb's local police force had systematically targeted African Americans.

US President Barack Obama said on Friday that he believes the racism revealed in the report was "not an isolated incident."