A six-year-old boy beat his newborn sister to death after their mother left them alone in a car, police have said.

Kathleen Marie Steele has been charged with aggravated manslaughter of a child following the shocking incident in Florida, USA.

Authorities said the 62-year-old left her three young children in the car while she went into a phone repair shop in St. Petersburg on Monday.

With the three children - the infant, a three-year-old and the six-year-old - alone in the car, the youngest began to cry, police said.

Pinellas County Sheriff Bob Gulatieri said the six-year-old began beating the baby, who was born last month, to make her stop crying

The baby, also named Kathleen Steele, was pronounced dead that evening having suffered head injuries.