A woman has been arrested in the US after allegedly locking her children, aged two and five, in the boot of her car while she went shopping.

US media report that Tori Castillo, 39, is facing child abuse charges and the children are now with their father.

Onlookers in the car park instructed the five-year-old on how to open the boot from the inside, reports say.

Passersby had noticed the car shaking and noise coming from inside where it was parked outside a Walmart.

The incident happened on Thursday in Riverdale, Utah. It is not clear how long the children were in the boot.

Local police officer Casey Warren told KTVX: "It's never right to lock a child inside of a vehicle, let alone a trunk of a vehicle.

"Obviously there's a lot of things that could go wrong."

He praised the "good Samaritans" who helped the children get out.

Under Utah state law, leaving a child under the age of nine unattended in a vehicle is punishable by a fine or jail time. The law was enacted in 2011 after repeated cases of children dying unattended in hot cars.