An eight-year-old boy beat a baby girl to death to stop her crying while her mother was out partying at a nightclub, US police said Wednesday, charging him with murder.

The boy was one of several children left alone for hours in a Birmingham, Alabama home last month, in what experts said was a rare case of a child so young being prosecuted.

"Since becoming a police officer over 22 years ago, this is by far one of the saddest cases I have witnessed in my career," Birmingham police spokesman Lieutenant Sean Edwards told AFP.