US police say a three-year-old Alabama boy has accidentally shot and killed his nine-year-old sister. Irondale police chief Ken Atkinson told that the shooting took place Saturday afternoon (6 February) at their grandparents' house in the Irondale suburb of Birmingham.

He said the boy found a loaded pistol on a bedroom nightstand and shot his sister in the head with it. The grandfather had left the loaded gun on the nightstand, not knowing his grandchildren were coming over that day, Atkinson said.

The police chief added that the incident appeared to be a "tragic accident" but an investigation is under way. Atkinson said detectives are interviewing the children's uncle and grandmother who were home at the time of the shooting, to learn exactly what happened.

The girl was flown with life-threatening injuries to Children's Hospital of Alabama, where she later died.

"The lesson is you have to make sure those weapons are put up, out of sight and out of reach, really of anyone, but especially children," he said. "It is tragic because this three-year-old has no idea what is going on right now. It is just a horrible situation."

Atkinson said he didn't know if the children were residents of Irondale.

In May 2015, a two-year-old boy died after shooting himself in the head in Virginia, and in April the same year a three-year-old boy killed a one-year-old in Cleveland after picking up an unattended gun.