A gunman opened fire at a supermarket in Buffalo city in the US state of New York on Saturday afternoon, killing at least 10 people, according to Buffalo Police Department.

The mass shooting took place at the Tops Markets on Jefferson Avenue. The gunman was an 18-year-old white male, said Buffalo Police Commissioner Joseph Gramaglia at a news conference.

He is under custody.

Gramaglia said 11 of the shooting victims were Black, while two others were white.

FBI is investigating the shooting as a hate crime and as an act of racially-motivated violent extremism.

The shooter was "very heavily" armed and was wearing tactical gear and a tactical helmet, according to Gramaglia.

"He had a camera that was live streaming what he was doing," he added.