US private firm SpaceX has aborted its launch of the Dragon capsule toward the ISS at the last second.

The last-second abort came when one of the Falcon 9's engines exceeded a technical limit that forced a shutdown of the launch attempt, which may be rescheduled as early as 22 May.

"Launch aborted: slightly high combustion chamber pressure on engine 5. Will adjust limits for countdown in a few days," SpaceX founder Elon Musk explained on Twitter.

A SpaceX spokesman said engineers would look into the causes, but that the issue was not believed to be something entirely new.

"We detected something was wrong with one of the limits" on one of the rocket's nine engines, said the spokesman on SpaceX's live broadcast of the event.

The next opportunity for launch is 3.44am on 22 May, according to NASA. 

The launch of the SpaceX Falcon 9 rocket, carrying the unmanned Dragon and over half a tonne of cargo toward the orbiting lab, will 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 US 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 programme 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 $390m so far of the total $680m SpaceX has spent on cargo development, Mr 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, SpaceX president Gwynne 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 $63m a ticket.

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