A three-year-old boy has shot dead his one-year-old sister in the US state of California, media reports said.

The incident was reported at 7:30am local time on Monday when the San Diego County Sheriff's Department received a call about a shooting at a home in Fallbrook, about 56 miles north of San Diego, reports Sky News.

Police said that deputies arrived and confirmed a toddler had managed to get hold of an "unsecured handgun" and the one-year-old had sustained a head injury.

The identities of the children have not been revealed so far.

The Sheriff's Department was quoted as saying by Sky News: "Fire Department personnel arrived on scene and transported her to Palomar Hospital. Despite lifesaving efforts, she was pronounced deceased at about 08:30 a.m."

"The Sheriff's Department extends our sympathies to the family and those affected by this death," read the statement.

"Sheriff's Homicide investigators are working to gather more information to determine the circumstances of the death. There are no outstanding suspects and there is no threat to the community. The investigation is ongoing," it said.