A black woman has been shot dead by police and her five-year-old son wounded by gunfire after an extended standoff at her flat in Baltimore, Maryland.

Korryn Gaines reportedly pointed her legally-owned pistol at police after they arrived at her home to arrest her for failing to appear at court for resisting arrest at a traffic stop earlier this year.

The 23-year-old uploaded live footage to Facebook showing portions of the five-hour standoff before it ended in gunfire.

Ms Gaines died while her son Kodi Gaines has been taken to a children's hospital and is in good condition, Baltimore County police said.

She has become the ninth black woman to be killed by US police this year while her death us likely to inflame Black Lives Matter protests nationally.

Officers had arrived at Gaines' flat to arrest her after she failed to appear in court on charges stemming from a traffic stop in March 10 this year but she refused to answer the door.

Police entered using a key obtained from the landlord and found Gaines inside armed with a gun which she pointed at them leading to an extended "barricade situation", police said.

The standoff ended after Gaines reportedly pointed her gun at one of the officers at 3pm and told him: "If you don't leave, I'm going to kill you."

Video reportedly posted by Gaines during the confrontation shows a young boy thought to be her son.

A woman's voice asks "Who's outside?" to which teh child answers "The police".

She then says: "What are they trying to do?" and he replies "Trying to kill us".

Police sad that the officers involved have been placed on administrative leave while they investigate the incident.

Meanwhile activists have been protesting the death of Gaines under both the Say Her Name campaign against police brutality and violence against black women.