Boeing's first astronaut flight on the company's Starliner capsule was scrubbed in the final minutes on Saturday.
The two NASA astronauts were strapped in for the journey to the International Space Station when the countdown stopped at 3 minutes and 50 seconds, the Associated Press reported.
With limited time to fix the problem, the flight was called off.
Technicians hurried to the launch pad at Cape Canaveral Space Force Station to remove astronautes Butch Wilmore and Suni Williams from the capsule above the Atlas V rocket.
The launch problem was not immediately revealed. NASA experts planned to discuss what went wrong at a press conference Saturday afternoon.
It's the second time Boeing and NASA has had to call off a launch attempt since May 6 when a flight was halted for rocket repairs.
The report said another attempt could come as soon as Sunday.
