A self-declared white supremacist pleaded guilty Monday to killing 10 Black people during a live-streamed shooting rampage at a supermarket in the US state of New York earlier this year.

Payton Gendron, 19, will spend the rest of his life in prison after admitting one state charge of domestic terrorism motivated by hate over the massacre in Buffalo in May.

Erie County District Attorney John Flynn said Gendron had pleaded guilty to all the charges against him, including 10 counts of murder in the first degree, three attempted murder charges and one count of criminal possession of a weapon.

Reporters in the courtroom said Gendron said the word "guilty" 15 times during Monday's hearing.

"While justice has been accomplished, nothing will ever bring back the 10 beautiful people who lost their lives on that day," Flynn told reporters.

"Hopefully the legal closure will provide the families and the victims some measure of relief," he added.

Gendron had planned the attack for months, targeting Tops Friendly Market in Buffalo because of the large American population in the surrounding neighborhood.

On May 14, the then 18-year-old drove from his hometown of Conklin, more than 200 miles (322 kilometers) away, with the intention of killing as many Black people as possible, prosecutors said.

Wearing heavy body armor and wielding an AR-15 assault rifle, Gendron shot four people in the store's parking lot, three of them fatally, before entering the supermarket.

Among those killed inside was a retired police officer working as a security guard. He fired several shots at the assailant before being shot and killed himself, police said.

Gendron wore a helmet with a video camera attached and live-streamed the two-minute attack on the platform Twitch.

The victims ranged in age from 32 to 86. Eleven of the 13 people shot were Black and two were white.

At one point, Gendron said "sorry" to a white man that he had injured and chose not to kill, "further demonstrating it was a racially motivated attack," Flynn said.

Police arrested Gendron within hours of the attack and investigators found a 180-page document on his computer that laid out his racist motivations for the massacre.

"It is clear this was an act of premeditated, pure evil," Buffalo Mayor Byron Brown said Monday.

Gendron is the first person in New York to be convicted of the state's domestic terrorism charge, which was introduced in 2020.

He is due to be formally sentenced on February 15 next year but the offense carries a mandatory penalty of life without parole.

Defense attorney Brian Parker told reporters that Gendron will not appeal the guilty pleas.

He still faces federal hate crime charges that could see him receive the death penalty.

Gendron had initially pleaded not guilty to a single count of first-degree murder before state authorities later added the other charges.