An unmanned rocket built and launched by private company SpaceX has exploded 139 seconds after lift off from the US state of Florida.

The rocket was supposed to have carried 1,800 kilograms of food, clothing, equipment and science experiments to those aboard the international space station (ISS), which now has four months of supplies remaining.

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At approximately 10:24am local time, those watching SpaceX’s live webcast saw the 63-metre long rocket Falcon 9 rocket burst into a huge puff of smoke, then chunks of debris fall like confetti from the blue sky.

“The vehicle has broken up,” NASA commentator George Diller said.

SpaceX owner Elon Musk, who is also boss of Tesla, explained later in the day that the explosion seemed to be linked to excessive pressure in the liquid oxygen tank.

Canadian astronaut Chris Hadfield posted online soon after that the accident had “serious ramifications” for the resupply of the ISS.

This was the first major disaster for Musk’s space company, whose competitors have been plagued by disasters.

Another private company, Orbital Sciences, lost one of its rockets in an explosion in October, and a Russian Progress supply ship was lost after lift-off in April.

NASA administrator Charles Bolden said the US space agency was “disappointed” at the loss but that the space station has “sufficient supplies for the next several months”.

A Russian Progress supply ship is scheduled to launch July 3, followed in August by a Japanese HTV flight, Bolden said.

Russian cosmonauts Gennady Padalka and Mikhail Kornienko and American astronaut Scott Kelly are currently living at the space station.

-with ABC, AAP.