The first attempt by a private company to send a cargo-loaded spacecraft to the International Space Station was delayed again on Wednesday and a new launch date remains uncertain, SpaceX said.

The planned May 7 launch has been postponed while engineers work out software issues, said SpaceX spokeswoman Kirstin Grantham.

"At this time, a May 7th launch appears unlikely," she said. "SpaceX is continuing to work through the software assurance process with NASA. We will issue a statement as soon as a new launch target is set."

Asked for further details by AFP, Grantham said May 10 was a possibility, but if not the delay may last a week or more. A new launch date was expected to be in place by Friday, she added.

SpaceX aims to be the first private company to send its own spacecraft to the orbiting research lab on a cargo mission.

The launch attempt has been repeatedly delayed, most recently from a planned April 30 launch date.

The Dragon spacecraft has also been built to carry humans to space, and the company, owned by Internet entrepreneur and PayPal co-founder Elon Musk, hopes that a successful cargo trip to the ISS will soon lead to a manned mission.

The end of the 30-year US space shuttle program last year left Russia as the sole nation capable of sending astronauts into orbit.

Japan, Europe and Russia's space agencies all operate supply ships that can bring cargo to the ISS, but never before has a private company launched its own craft capable of toting both cargo and humans.

SpaceX made history with its Dragon launch in December 2010, becoming the first commercial outfit to send a spacecraft into orbit and back.

SpaceX and several other companies are competing to be the first to operate a private capsule that could tote astronauts and cargo to the ISS.

In addition to SpaceX, Orbital Sciences also has a NASA contract to deliver freight to the space station.

Other companies in the private space race include aerospace giant Boeing, the Nevada-based Sierra Nevada Corporation, and Washington state-based BlueOrigin LLC.

The main goals of SpaceX's cargo flight include a fly-by of the ISS and a berthing operation in which the company's reusable spacecraft, the Dragon, will approach the ISS and the crew aboard the orbiting outpost will use the ISS robotic arm to help it latch on.

The gumdrop-shaped Dragon capsule will carry 521 kilograms (1,148 pounds) of cargo for the space lab and will also aim to return a 660 kg (1,455 lb) load to Earth.

Speaking to reporters in mid-April, Musk voiced cautious optimism that the attempt would work, noting that the company has launched the Falcon 9 rocket twice before and successfully sent its Dragon capsule into orbit and back once.

"I think we have a got a pretty good shot but it is worth emphasizing that there is a lot that can go wrong on a mission like this," Musk said.

If this attempt does not work out, SpaceX will try again, he said.