The California-based company SpaceX on Saturday scrubbed the launch of its Dragon capsule toward the International Space Station at the last second due to a rocket engine problem.

The abort came a half-second before liftoff due to high pressure in the center engine of the Falcon 9 rocket, forcing a shutdown of the launch attempt. The next try is expected May 22 at 3:44 am (0744 GMT).

"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 the launch was scrubbed.

The postponement came when an engine controller noted high chamber pressure in engine five of the rocket, which requires all nine engines for a successful liftoff.

Inspectors would be searching for a root cause to the problem later Saturday, she said.

A similar issue with engine five forced a temporary delay of the Falcon 9's first-ever flight, but that liftoff was not scrubbed because there was a longer launch window and SpaceX was able to recycle the attempt, Shotwell said.

However, this time there was a very narrow window of opportunity to launch toward the ISS so the attempt was put off.

"We will be out there looking for whatever we can find and we will put out a statement as soon as we find a root cause," Shotwell said, adding that early indications have ruled out a sensor failure or a faulty fuel valve.

The launch of the SpaceX Falcon 9 rocket, carrying the unmanned Dragon and over half a ton of cargo toward the orbiting lab, would mark the first attempt to send a privately built spacecraft to the research outpost, where it plans to do a fly-under followed by a berthing.

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 could come up with a replacement.

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

SpaceX and Orbital Sciences Corporation both have billion-dollar contracts with NASA to supply cargo to the ISS in the coming years, and they get NASA funds in exchange for meeting key milestones in their projects.

NASA has given SpaceX about $390 million so far of the total $680 million SpaceX has spent on cargo development, Shotwell said.

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.

With seven seats aboard the Dragon capsule, she said SpaceX could someday offer that to NASA for $140 million per mission -- about $20 million per seat.