The grandfather of a two-year-old girl, accidentally shot and killed by her four-year-old cousin in Southern California, has been arrested.

Cesar Lopez (53) was taken into custody on Friday on suspicion of child endangerment and being a felon in possession of a firearm, officials said.

The shooting happened at around 9.15am in the Muscoy area, east of Los Angeles.

The wounded toddler was taken to hospital but died about an hour later, officials said.

According to several US media reports, the boy allegedly involved in the shooting was visibly distraught as he was taken away from the home.

Investigators said Lopez had left the gun in a place that was accessible to children.

They confirmed that both children lived at the house and there were adults in the property at the time of the tragedy.

Several neighbours described the girl's death as "devastating" and urged gun owners to lock up their weapons so children could not get hold of them.

According to the Gun Violence Archive, an independent data collection and research group, there have been 7,965 gun-related deaths in the United States this year so far - including 380 children under age of 11.