A five-year-old boy in the US state of Kentucky accidentally shot his two-year-old sister dead with a rifle he had received as a gift last year.

The children's mother was home at the time yesterday afternoon, but had stepped out to the front porch for a few minutes and "she heard the gun go off," said Cumberland County Coroner Gary White.

He said the rifle was kept in a corner and the family did not realise a bullet was left inside it.

Mr White told the Lexington Herald-Leader newspaper the boy received the .22-calibre rifle, especially made for children, as a gift.

He identified the gun as a Crickett, referring to a company that specifically makes guns, clothes and books for children.

The coroner said: "It's a little rifle for a kid ... The little boy's used to shooting the little gun.

"Down in Kentucky where we're from, you know, guns are passed down from generation to generation. You start at a young age with guns for hunting and everything.

"Accidents happen with guns. They thought the gun was actually unloaded and it wasn't."

Mr White said the girl died of a single gunshot wound to the chest area.

It is not clear whether any charges will be filed, said Kentucky State Police spokesman Trooper Billy Gregory.

Keystone Sporting Arms produced 60,000 Crickett and Chipmunk rifles in 2008, according to its website.

It also makes guns for adults. The smaller guns come in all sorts of colours, including blue and pink.

The company's slogan is "my first rifle," and its website has a "Kids Corner" section with photos of children at shooting ranges and on bird and deer hunts.