US police has shot dead another black man in Washington DC amid the latest round of popular protests against police brutality.

The man, said to be aged 18, was shot in the southeast of the city as police officers were following up on a tip about guns allegedly stashed in a vehicle.

"When they approached the vehicle, some of the folks of the vehicle fled on foot. One of the officers discharged their firearm," Washington police said.

"We have what we believe to be an adult male that was taken to a local hospital and pronounced dead."

The shooting came in the midst of months-long protests against police brutality across the United States, and just two days after Los Angeles police officers shot dead an African-American man.

The victim was riding his bike when LA police tried to stop him.

A Washington council representative told news site DCist said, “It's tragic. The young man just celebrated his 18th birthday."

"We don't know a lot about the situation. But, in my mind, there are very few reasons for a 17-year-old or 18-year-old young boy to be killed in our city, especially if this man was unarmed or if he was fleeing."

The US has been the scene of fresh anti-racism protests in recent days following the police shooting of Jacob Blake, a 29-year-old black man, in front of his children during an arrest on August 23. Blake has been left paralyzed.

Kyle Rittenhouse, a Trump supporter, shot three protesters, two fatally, with an assault rifle in Kenosha on the third night of protests against police violence.

The 17-year-old vigilante has been charged with two counts of first degree murder and attempted murder.

Democrats and police reform advocates see Kenosha as a symbol of institutional racism leading to deadly encounters between officers and black suspects, AFP said.

Democratic presidential nominee, Joe Biden, has repeatedly condemned disorder at demonstrations, and has defended peaceful protesters, arguing justice must be served in cases of "unwarranted police violence." He called for rioters and looters to be prosecuted.

The 77-year-old Democrat accused Trump of fomenting violence after a week of deadly unrest.

"Fires are burning and we have a president who fans the flames rather than fighting the flames," Biden was speaking in Pittsburgh in the swing state of Pennsylvania.

The protests over police brutality and racial inequality that have gripped American cities for months ignited after the killing of a black man, George Floyd, by a Minneapolis police officer in May.

President Donald Trump, who has been trailing Biden in opinion polls ahead of the Nov. 3 election, has been appealing to his base of white supporters with a "law and order" message.

Trump signed a memo on Wednesday that threatens to cut federal funding to "lawless" cities, including Seattle, Portland, New York and Washington.