Los Angeles  - A six-year-old boy took off alone in an experimental balloon Thursday, which sped for two hours over the Colorado landscape as rescuers struggled with a plan to bring the flying-saucer-shaped 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.

According to a spokesman for the Larimer County Sheriff's office, the six-year-old and his older brother were playing outside when the child got into the balloon and it took off.

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

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.

Prior to the landing the mylar-covered balloon was losing helium as police and military officials examined ways to bring the balloon under control.  dpa