Engineers aborted the launch of a privately built spacecraft on a landmark mission to the International Space Station at the last second Saturday due to a rocket engine problem.

The California-based company SpaceX scrubbed the launch of its Dragon capsule a half-second before liftoff after an engine controller noted high pressure in the central engine of the Falcon 9 rocket, forcing the shutdown.

"This is not failure. We aborted with purpose. It would be a failure if we were to have lifted off with an engine trending in this direction," SpaceX president Gwynne Shotwell told reporters.

After inspection, SpaceX engineers found the cause of the high pressure in the combustion chamber was due to a faulty valve.

"We have discovered root cause and repairs are underway," the company said in a statement.

"During rigorous inspections of the engine, SpaceX engineers discovered a faulty check valve on the Merlin engine. We are now in the process of replacing the failed valve. Those repairs should be complete tonight."

More data would be reviewed on Sunday, and "if things look good, we will be ready to attempt to launch on Tuesday, May 22nd at 3:44 am (0744 GMT)."

If Tuesday's launch window cannot be met, another opportunity opens up on Wednesday at 3:22 am (0722 GMT).

NASA urged patience for what would mark the first attempt to send a privately built spacecraft to the orbiting research outpost.

"We're ready to support when SpaceX is ready to go," said Alan Lindenmoyer, NASA's manager of the Commercial Crew and Cargo Program.

SpaceX is the first of several US competitors to try sending its own cargo-bearing spacecraft to the ISS with the goal of restoring US access to space for human travelers by 2015.

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

Until now, only the space agencies of Russia, Japan and Europe have been able to send supply ships to the ISS.

The United States had that capacity too, with its iconic space shuttle that long served as part astronaut bus, part delivery truck for the lab.

But the 30-year shuttle program ended for good in 2011, leaving Russia as the sole taxi for astronauts to the ISS until private industry comes up with a replacement.

SpaceX has benefited from NASA dollars in its quest but has also poured its own money into the project.

SpaceX and Orbital Sciences Corporation both have billion-dollar contracts with NASA to supply cargo to the ISS in the coming years.

The US space agency has given SpaceX about $390 million so far of the total $680 million that the California-based company has spent on cargo development, according to Shotwell.

SpaceX also gets funding from NASA on a separate effort to develop a commercial crew vehicle for carrying astronauts to space, along with competitors Blue Origin, Boeing and Sierra Nevada.

In a few years' time, Shotwell said she hopes SpaceX will be able to undercut the hefty price NASA pays Russia for US astronauts to get a seat aboard the Soyuz space capsule -- around $63 million a ticket.