SpaceX on Sunday postponed for 24 hours the launch of a secretive US government payload, known only as NROL-76, due to a "sensor issue" with the rocket, a spokesman said.
"Out of an abundance of caution we have decided to scrub today's launch," a SpaceX spokesman said, describing the issue as relating to the first stage of the Falcon 9 rocket.
Another opportunity for launch opens Monday at 7:00 am (1100 GMT).
The payload for the National Reconnaissance Office, which makes and operates spy satellites for the United States, will be the first military launch for the California-based aerospace company headed by billionaire tycoon Elon Musk.
"As a matter of policy and because of classification, NRO does not provide information about our contracts," a spokeswoman told AFP.
Until now, the US military has spent billions per year exclusively with United Launch Alliance, a joint operation of aerospace giants Boeing and Lockheed Martin, to launch government satellites.
SpaceX in 2014 protested the US Air Force's practice of using only ULA, saying it unfairly awarded billions of dollars to a single company for national security launches.
SpaceX was selected to launch NROL-76 "after a competition," said the NRO spokeswoman.
She said she did not know when the contract was awarded. It was first announced last year.
SpaceX regularly launches unmanned cargo ships to the International Space Station, and is working on a crew capsule that could carry humans into orbit as early as next year.