Image copyright KHTV Image caption Acen King and his grandmother "were completely innocent", said police

A three-year-old boy being taken out shopping by his grandmother has been killed in a road rage shooting in Little Rock, Arkansas, US police say.

Their vehicle was fired upon by another driver who thought the woman had not driven quickly enough through a stop sign, police said.

The grandmother drove away but the boy later died in hospital.

Police are hunting for a black Chevrolet Impala but gave few details of the man driving it.

Authorities identified the grandmother as 47-year-old Kim King-Macon and the boy as Acen King, according to local media reports.

Image copyright AP Image caption The boy's grandmother phoned police from near a shopping centre

The boy and his grandmother "were completely innocent" and had no relationship with Saturday's gunman, Little Rock Police tweeted.

"This is probably one of the most frustrating things you will encounter in your career when it involves children, particularly of kids that are [at] this age very innocent, they can do very little to protect themselves," said Chief Kenton Buckner.

The victim was struck at least once, police said. Ms King was initially unaware he had been shot when she drove off, later calling police from a shopping centre.

Last month, a two-year-old girl was killed in Little Rock when someone drove by and fired into her vehicle. The perpetrator is yet to be found.