Three girls plunged 45 feet from a defective ferris wheel at a Tennessee fairground, leaving one of them in critical condition with a traumatic head injury.

The youngest of the victims, a 6-year-old, suffered the head injury and had to be intubated at the scene before she was airlifted to hospital, officials said.

"She has been under the continual care of the pediatric surgery service, the pediatric intensive care unit team as well as the neurosurgery team," said Dr Bracken Burns, director of trauma services for Johnson City Medical Center.

The girl's sister, aged 10, suffered injuries to her forearms and was in a cast. A third victim, 16, was initially in critical condition but her status has been upgraded to stable.

The horrifying accident took place on Monday (8 August) at the Greene County Fair in Greeneville when a basket carrying the three girls suddenly overturned and spilled them to the ground. Early inspection reports point to a mechanical failure, local News Channel 11 reported.

"I was hollering, 'Please stop! Stop! Stop the machine!' It seemed like it was a millennium before it stopped," witness Gregory Lynthacum told ABC News. "I'd seen blood coming from their mouth. I actually thought they were dead. It was so horrific."

All other rides at the fair were also inspected, but only minor issues were found and were corrected on the spot. "I spent a long, hot day today looking at all the other rides out here," an inspector said at a press conference at the scene.

"Other than the ferris wheel that was involved with the accident, and I've done a detailed inspection of each and every ride ... nothing serious [was] wrong with any of the rides. There was some minor items, such as sanding fiber glass, some minor cracks that needed to be cleaned up and smoothed."

The ferris wheel will be sent back to the manufacturer for a full work-up. "We'll be relying heavily on the inspector to know whether there was mechanical issues. If there was neglect or anything like that," said Captain Tim Davis with the Greeneville Police Department.

The Tennessee incident happened a day after a 10-year-old Caleb Thomas Schwab was killed while riding a Kansas waterslide, billed as the world's tallest. Both incidents are raising questions about the safety of amusement rides and how they are regulated.