An 11-year-old boy left at home with his four-year-old sister fatally shot a teenage intruder who attempted several times to get into their St Louis house, police in the US have said.

Police were called to the house on Thursday and found the body of a 16-year-old lying in the front foyer.

The teenager, whose identity has not been released, had been shot once in the head as he attempted to break into the home.

Sergeant Brian Schellman said it was the third time the intruder had attempted to break in, and he was fatally shot after managing to get inside.

Authorities have said a second suspect fled the scene but was arrested a short time later. That suspect has not been identified.

Sgt Schellman said the children's mother was not at home during the attempted break-in, but is co-operating with the investigation.

It was unclear how the 11-year-old had access to a firearm.

But the boy's mother is reported to have bought the handgun after previous attempted break-ins.

Neighbours also claimed the two suspects had been sitting on the porch and shouting before the shooting, CNN reported.

Both the boy and his younger sister were unhurt.