CAPE CANAVERAL, Fla. — A potentially dangerous hydrogen gas leak cropped up during the fueling of space shuttle Endeavour on Saturday morning and forced NASA to postpone the launch by at least four days.

It was almost identical to a leak that stalled another flight back in March and threatened to bump Endeavour's space station construction mission all the way into July.

NASA halted the countdown shortly after midnight, less than seven hours before Endeavour was due to blast off. The seven astronauts had yet to suit up.

Launch director Mike Leinbach said the leak, located at a vent line hookup on the fuel tank, was significant. Hydrogen gas is extremely volatile and can burn in large enough quantities, he noted.

"There's no way we could have continued," Leinbach said at a news conference. "It's a commodity you just don't mess with."

NASA's launch team immediately began draining Endeavour's external fuel tank while trying to figure out what went wrong.

NASA is up against a tight deadline. A four-day delay would make Endeavour's next launch attempt Wednesday. But that's the day the space agency is supposed to launch a moon-bound spacecraft aboard an unmanned rocket.