Miami:SpaceX on Thursday postponed the launch of a secretive US government payload known as Zuma, a mission whose nature -- and the agency behind it -- is a mystery.

"We have decided to stand down and take a closer look at data from recent fairing testing for another customer," SpaceX said in a statement issued late Thursday about two hours before the planned launch window was to open. The fairing is the nose cone part of the rocket that protects the payload.

"Though we have preserved the range opportunity for tomorrow, we will take the time we need to complete the data review and will then confirm a new launch date." SpaceX and the Pentagon did not respond to requests for comment about the nature of the mission.

Northrup Grumman, the maker of the payload, said it was for the US government and would be delivered to low-Earth orbit, but offered no other details.

SpaceX is no stranger to national security launches.

Earlier this year the California-based company headed by billionaire entrepreneur Elon Musk launched a spy satellite for the National Reconnaissance Office, and an X-37B space plane for the US Air Force.

After liftoff, SpaceX will attempt to return the first stage portion of its Falcon 9 rocket to Earth for a controlled landing on solid ground near Cape Canaveral.

If successful, it will be the 20th upright touchdown for a Falcon 9 since the company began its effort to recycle costly rocket parts and make spaceflight more affordable.-AFP