Last Updated Jan 6, 2015 10:32 AM EST

Launch of a SpaceX Falcon 9 rocket carrying a Dragon cargo ship loaded with more than 5,100 pounds of equipment and supplies bound for the International Space Station was called off Tuesday less than two minutes before liftoff because of apparent problems with the second stage engine steering system.

Trouble with a second stage actuator assembly apparently cropped up last month during or in the wake of a first-stage engine test firing, sources said. The test firing was cut short, presumably because of a problem with the first stage propulsion system. SpaceX, in keeping with company policy, did not provide any details.

In any case, launch eventually was delayed three weeks to allow time for a second test firing, to give the SpaceX launch team a break for the Christmas holidays and to allow temperature constraints related to the station's orbit to improve.

Sources said the suspect actuator assembly, one of two used to move the second stage engine nozzle for steering, was examined and given a clean bill of health. The Falcon 9's first- and second-stage systems performed normally during the second countdown and test firing, setting the stage for Tuesday's launch try.

A SpaceX spokesman said Tuesday, after the abort, that he could not address the engine test firing issue or provide any details about the second-stage steering system actuator. A senior SpaceX manager, asked about the test firing problem during a news conference Monday, did not provide any additional details other than saying the issue, whatever it was, had been resolved.

In a brief statement Tuesday, the company spokesman said only that during the terminal countdown "engineers observed drift on one of the two thrust vector actuators on the second stage that would likely have caused an automatic abort. Engineers called a hold in order to take a closer look."

Company founder Elon Musk said in a Twitter posting: "Need to investigate the upper stage Z actuator. Was behaving strangely. Next launch attempt on Friday."

Launch preparations went smoothly early Tuesday and the countdown ticked cleanly through fueling operations toward a planned liftoff at 6:20 a.m. (GMT-5). There were no known technical problems and the weather cooperated with fair conditions.

Then at T-minus one minute and 21 seconds, a SpaceX controller called a hold on the countdown audio net. To reach the space station, the rocket had to launch almost directly into the plane of the lab's orbit, an "instantaneous" launch window that left no margin for delay.

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SpaceX scrubs space station supply flight at last minute