A six-year-old girl has died after her eight-year-old brother crashed the family car in the US state of Arizona. The girl was found dead in the passenger seat of the car, wearing her pyjamas, police said.

The boy suffered ankle injuries after the family's Hyundai Elantra hit a pole at the side of the road, one block from their home in south Phoenix. Police had spotted the vehicle moments earlier and were attempting to pull it over when the accident happened.

It is understood the children's mother raised the alarm after believing they had been kidnapped at 10pm on Wednesday night.

A frantic 911 call records the mother pleading with officers to find her children after they disappeared. She had previously bathed the children and put them to bed, said police sergeant Steve Martos, of the Phoenix Police Department.

"It's a tragic story, all the way round," said Mr. Martos, who did not give the children's names.

Police said they started to get numerous 911 calls from around 10.15pm. One caller said the car had struck a fence. Another, who followed the vehicle, reported that children were inside.

At around 10.30pm, when police tried to pull the vehicle over, it veered to the right and hit a pole, Mr. Martos said.

"This was one of those situations when the officers believed they were trying to rescue kids from a possible kidnapper," the police officer added.

"When they ran up to the vehicle, they found an eight-year-old in the driver's seat, and a six-year-old in the passenger seat, severely injured."

Both the children were in their pyjamas and the girl was not wearing a seat belt. The air bags did not deploy.

The mother had walked to the scene and discovered that her children were in the car. Police said they did not know what prompted the boy to take the car.

They have not spoken to him, but no criminal complaints are expected to be filed because he is too young.