The countdown was halted with just two minutes remaining until liftoff from Kennedy Space Center. With just a split second to blast off, there was no time to deal with the problem, which involved the engine ignition system.

SpaceX did not immediately say when it would try again. The next attempt could come as early as Tuesday, although poor weather was forecast up the East Coast in the emergency recovery area.

Strapped into the capsule atop the Falcon rocket were two NASA astronauts, one Russian cosmonaut and one astronaut from the United Arab Emirates. They had to wait until all the fuel was drained from the rocket — an hourlong process — before getting out.

“We’ll be sitting here waiting,” commander Stephen Bowen assured everyone. “We’re all feeling good.”

Bowen and his crew — including the first astronaut from the United Arab Emirates assigned to a monthslong mission, Sultan al-Neyadi — will replace four space station residents who have been up there since October.

Officials said the problem involved ground equipment used for loading the engine ignition fluid. The launch team could not be sure there was a full load. A SpaceX engineer likened this critical system to spark plugs for a car.