An unmanned SpaceX rocket has exploded less than three minutes after lift-off from Cape Canaveral, Florida, in the first major disaster for the fast-charging company headed by internet tycoon Elon Musk.

Sunday's accident was the third in less than a year involving US and Russian supply ships bound for the International Space Station, and raised new concerns about the flow of food and gear to the astronauts living in orbit.

Skies were sunny and clear for the 10.21am launch of the gleaming white Falcon 9 rocket that was meant to propel the Dragon cargo ship on a routine supply mission, the seventh for SpaceX so far.

But two minutes, 19 seconds into the flight, contact was lost. Live television images from SpaceX's webcast and NASA television showed a huge puff of smoke billowing outward for several seconds, then tiny bits of the rocket falling like confetti against a backdrop of blue sky.

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

SpaceX's live webcast of the launch went silent as the rocket exploded.

Moments later, a SpaceX commentator said the video link from the vehicle had been lost.

"There was some kind of anomaly during first stage flight," the commentator said, noting that the rocket had ignited its nine Merlin engines and reached supersonic speed.

Later, on Twitter, Musk said the Falcon 9 "experienced a problem shortly before first stage shutdown," referring to the phase of flight before the cargo ship would have been able to separate from the first stage of the rocket and reach orbit.

The problem appeared to be linked to excessive pressure in the liquid oxygen tank, Musk wrote.

"Data suggests counterintuitive cause," said Musk, a lifelong space enthusiast who also heads Tesla Motors.

"That's all we can say with confidence right now."

The loss came as a surprise to many who have followed Musk's California-based company through more than a dozen successful launches.

Competitor 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.

The Dragon cargo ship was carrying 1800kg of gear to the space station, including a large parking space, known as an International Docking Adaptor, designed to make it easier for an array of commercial crew spacecraft to dock at the orbiting lab in the future.

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.

"Sadly failed. Space is hard," Kelly said on Twitter, posting a picture of his view of the Florida coast from space.