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

It was almost identical to a leak that stalled a flight back in March and threatened to bump Endeavour's construction mission at the International Space Station 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 said.

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

The hydrogen gas leak is similar to one that NASA faced while trying to launch Discovery three months ago. That flight was delayed four days because of the problem and shortened as well. Atlantis, however, encountered no such trouble during its countdown in May for the Hubble Space Telescope repair mission.

NASA's launch team immediately began draining Endeavour's external fuel tank while trying to figure out what went wrong. In March, the leak occurred where a vent line hooks up to the tank. The hookup was replaced along with a couple of seals and the seepage stopped, but engineers never did determine the exact source of the trouble.

Officials said workers would not be able to get to the vent line on Endeavour's tank until today.

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.

Mike Moses, chairman of the mission management team, said it was too soon to say which mission would take priority. "We haven't even begun to work that yet," he said.

If Endeavour isn't flying by Saturday, it will have to wait until July 11 for the next launch attempt because of unfavorable sun angles that would make the shuttle too hot while docked at the International Space Station.