A three-year-old boy picked up an unattended gun inside a home and it went off, shooting a one-year-old boy in the head and killing him, US police say.

Cleveland Police Chief Calvin Williams told reporters gathered outside the home after the shooting on Sunday afternoon that investigators were trying to determine where the gun came from.

The home in the US city of Cleveland where a 3-year-old boy picked up an unattended gun and shot dead a one-year-old. Credit:AP

Cleveland.com reported the one-year-old was taken to a hospital and pronounced dead.

Full details about the shooting on the city's east side weren't released, but Mr Williams said at least one adult was home when it happened.