Arkansas police have launched a manhunt after a toddler was shot and killed in a road rage incident.

Three-year-old Acen King was shot while riding in car with his grandmother on a shopping trip after the driver behind their vehicle opened fire.

Police said the boy and his grandmother were at a stop sign in south west Little Rock on Saturday when a tall African American man driving behind them got out of his car and opened fire because the woman “wasn’t moving fast enough”. A one-year-old baby was also in the car.

The boy was struck by gun fire at least once but his grandmother did not realise he had been hit until they pulled into the car park of a shopping mall nearby, CNN reports. She called emergency services but the boy was later pronounced dead at hospital.

Police are searching for the man and his vehicle, described as an older black Chevrolet Impala. A $40,000 (£32,400) reward is being offered for the arrest and conviction of the man, with $20,000 offered by police and $20,000 from the city of Little Rock.

Lieutenant Steve McClanahan said investigators believed the grandmother and the boy "were completely innocent" and had no relationship with the shooter.

Little Rock Police Department chief Kenton Buckner told KARK: “I certainly have no information to say that anyone in this family has done anything to cost this young person’s life. But this is about as frustrated as you can be as a public safety official or just a plain citizen.”

Police said the incident marks the 40th homicide this year.