Los Angeles  - Authorities were searching Colorado's countryside and suburbs Thursday for a six-year-old boy who reportedly took off alone in an experimental balloon Thursday, but was nowhere to be found when it landed two hours later.

The flight of the flying-saucer shaped balloon captivated US broadcasters as it drifted over the Colorado landscape while rescuers struggled with a plan to bring the helium-filled craft under control.

Live television pictures showed the balloon land softly in a ploughed field, but police said there was no child in the balloon, boosting speculation that the child may have fallen out of the balloon at some point.

The drama started about 11 am local time when the six-year-old, named as Falcon Heene, and his older brother were playing outside when the child got into the balloon and it took off.

His father, Richard Heene, was well known as a daredevil stormchaser and amateur scientist. The family was also featured on the TV reality show Wife Swap, in which which the mothers of two often opposite families switch places for two weeks. Falcon's mother, Mayumi Heene, switched roles with a mother who criticized the family's allegedly lax safety standards for the children.

Amateur photographs appeared to show the balloon flying high above some houses with what appeared to be a small box falling from it. Local reports quoted the family as telling law enforcement officers that a small box suspended from the silver balloon by string was not attached to the craft when it made its crash landing.

But for the duration of the balloon's flight authorities had been convinced that the boy was inside the fast-moving craft.

"This balloon was never meant to actually carry anybody," a sheriff's spokesman told reporters earlier. "The little compartment where their son is in is very small and not very well attached."

Local TV station KUSA said at one point that the helium-filled balloon was flying at around 30 kilometres per hour at a height of 2,500 metres. It travelled about 80 kilometres before it landed in an empty field. (dpa)