Technical issues with the rocket, Falcon 9, caused the company to postpone the demonstration launch from Tuesday, according to SpaceX president Gwynne Shotwell.

"It looks like the first attempt for this flight is no earlier than Thursday," Shotwell told a press conference.

Inspectors determined early Monday that "indications in the weld joint were such that we wanted to take additional steps to look at it," said Shotwell, saying the issues involved "porosity and potentially cracking in a weld joint."

If the nozzle needs to be replaced, the launch could be set back to Friday, she said.

The Dragon spacecraft, which is unmanned for now but is designed with seats for seven and an ample cargo hold, aims to hurtle into orbit and then splash into the Pacific Ocean about four to five hours later, the company said.

The bullet-shaped space capsule, which could one day tote supplies to the International Space Station, is scheduled to blast off from Cape Canaveral in Florida.

The operation hopes to showcase the capsule's ability to launch and separate from the Falcon 9 rocket, orbit Earth, transmit signals and receive commands, and then re-enter the Earth's atmosphere for an ocean recovery.

Never before has a non-government owned spacecraft successfully pulled off such a feat and the operation carries significant risks.

Among them, the craft must maneuver in orbit at speeds of more than 17,000 miles (27,000 kilometers) per hour, survive a fiery re-entry into orbit and manage a safe parachute landing into the ocean.

SpaceX said in a statement that the Dragon, which has no wings unlike the space shuttles, will control its re-entry through "onboard Draco thrusters which enable the spacecraft to touchdown at a very precise location -- ultimately within a few hundred yards (meters) of its target."

In late November, the Federal Aviation Administration issued its first license to SpaceX to permit a privately owned spacecraft to re-enter Earth's orbit.

A previous flight by the world's first commercial spaceship, owned by Virgin Galactic's Richard Branson, made its first piloted journey in October but stayed close to its California base and did not enter orbit.

If the Dragon works, the next steps are for a fly-by of the ISS as part of a five-day mission in which the Dragon will approach the orbiting station within six miles (10 kilometers), and later an actual cargo and crew mission to the ISS.

Both are scheduled to take place in 2011.

As tall as an 18-story building, the Falcon 9 rocket that will carry the Dragon was successfully test launched in June.

The US space agency NASA signed a 1.6-billion-dollar contract with SpaceX in December 2008 under the Commercial Orbital Transportation Services (COTS) program to provide twelve spacecraft with cargo capacity of at least 20 tonnes to resupply the International Space Station (ISS) through 2016.

NASA has also signed a contract of 1.9 billion dollars with Orbital Space Corporation for eight launches of its Taurus II rocket starting in 2011.

President Barack Obama hopes the private sector will help fill the gap that will open when the space shuttle fleet is retired next year, and before a new generation of spacecraft is developed.

The three US shuttles -- Discovery, Atlantis and Endeavour -- will become museum pieces after a final shuttle mission to the space station in 2011.

Obama has proposed spending six billion dollars over five years to help the private sector develop reliable and affordable launchers to transport cargo and US astronauts to the International Space Station.

During the transition period, the United States will depend on Russian Soyuz rockets for access to the ISS.

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