US authorities have said that they did not find a six-year-old boy believed to be trapped inside a home-made helium balloon that floated for more than two hours across Colorado before crash landing.

The fate of the child was not immediately clear after security officials said that no one was inside the balloon, which landed in a field on Thursday.

Authorities are searching for the child on the ground, including in the area where he lives.

Cathy Davis, a Larimer County Sheriff's department spokeswoman, said that the the boy and his brother were playing outside when the older boy saw the younger one go into a compartment at the bottom of the balloon before it broke loose from its tether.

'Afternoon drama'

Al Jazeera's Rosiland Jordan in Washington said: "This has turned from an afternoon drama on the cable television news channels into now perhaps a criminal investigation.

"Authorities say they searched the family's home when they first got the report that the child was missing, but when you're considering that we're only talking about a span of three hours or so, you have to wonder how thorough was this search, have they really looked at all the possibilities about the whereabouts of this boy?"

The balloon was capable of flying to an altitude of about 3,000 metres, a height where oxygen is in very short supply.

Helicopters followed the silver balloon, which was reportedly made by the boy's father.

The balloon's basket was enclosed, making it difficult to asceratin whether anyone was trapped inside.