DENVER, COLORADO - A helium balloon that flew out of control over Colorado landed on Thursday but a 6-year-old boy who had been thought to be on board was not in the craft, local media said.

It was not known where the boy was.

The boy, who was not identified, was reported to have climbed into the small homemade craft at his family's home before it floated away and across the Colorado skies at up to 7,000 feet (2,100 metres).

Television images showed the silver ballon, resembling a flying saucer and about 20 feet (six metres) across, floating above Colorado before it landed, as authorities scrambled to find a way to bring it down safely.

A Larimer County Sheriff's spokeswoman described the craft as 'a homemade helium balloon made to look like a UFO.' She said the boy climbed into the balloon at his family's home in Fort Collins.

The craft then apparently broke away from a pair of tethers holding it to the ground and floated away.

Aviation experts said that the boy could survive the flight but would be cold and possibly disoriented at high altitudes.