A three-year-old boy in a car with his grandmother has been killed in an road rage shooting when a driver opened fire because he thought she "wasn't moving fast enough".

Police said the boy and his grandmother, who were on a shopping trip, were at a stop sign in south-west Little Rock, Arkansas, on Saturday when a driver apparently angry about the delay stepped out of his car and opened fire. The boy was struck by gunfire at least once.

The grandmother, who was unhurt, drove away and called police from a shopping centre.

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

Police are looking for an older black Chevrolet Impala driven by a tall black man.

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