Witnesses in the area reported feeling a shock wave in buildings several miles away and seeing a plume of smoke rising from Launch Complex 40 just after 9 a.m.
Officials at SpaceX, the Air Force's 45th Space Wing and Kennedy Space Center could not immediately confirm what happened or whether there were any injuries.
The rocket was believed to be performing a test-firing of its nine Merlin main engines as a standard check of their readiness for launch, and so the area probably was cleared for that hazardous operation.
The rocket was scheduled to launch the Amos-6 communications satellite for Israeli company Spacecom at 3 a.m. Saturday.
The condition of the satellite could not immediately be confirmed.
Facebook planned to use some of the satellite's capacity to expand its Internet.org initiative in Africa.
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