Space Exploration Technologies Corp. (SpaceX) and Rocketplane Kistler (RpK) were chosen among six finalists and will have to demonstrate they can fly such missions before NASA can offer contracts, officials said.

"We have a definite need at NASA," Scott Horowitz, NASA's associate administrator for the Exploration Systems Mission, told reporters.

"We are a customer that needs after we retire the shuttle to be able to re-supply the space station," he said.

"We need to be able to send crew and cargo to the International Space station. If the commercial sector can do it safely and reliably and most cost effectively then it's our best interest to buy service," Horowitz said.

The National Aeronautics and Space Administration has a 400-million-dollar budget for the project, dubbed Commercial Orbital Transportation Services (COTS). More than 20 companies had competed for the job before NASA narrowed the candidates to six and two.

NASA plans to launch 16 shuttle missions to the ISS over the next four years to complete construction of the half-finished orbiting laboratory.

Atlantis is scheduled to lift off August 27 in the first ISS construction mission since the 2003 Columbia shuttle disaster.

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