CAPE CANAVERAL, Fla. - For the second time in less than a week, a potentially dangerous hydrogen gas leak early yesterday forced NASA to delay Endeavour's launch to the International Space Station, this time until July at the earliest.

Launch officials waited almost an hour after the leak appeared during fueling, trying to fix it through remote commands, before calling off the predawn launch.

The leak occurred in the same place as one that cropped up Saturday, in the hydrogen gas vent line that hooks up to the external fuel tank. A similar problem stalled a shuttle flight three months ago.

"We're going to step back and figure out what the problem is and go fix it," said LeRoy Cain, deputy space shuttle program manager. "And then we'll fly as soon as we're ready to safely go do that."

Mission managers had ordered repairs after Saturday's delay. The hookup itself and two seals were replaced. The same repair worked in March, but engineers never found the cause of the problem.

Even before hydrogen gas began leaking - a serious situation because of its flammability - NASA was up against a tight deadline for making the 5:40 a.m. launch. Fueling was delayed three hours by thunderstorms Tuesday night, and the launch team was racing against the clock to catch up.

The seven astronauts were still in crew quarters when the leak was detected. It was a keen disappointment considering they had just gotten a chance, with the start of fueling, at making the launch.

"It's a reminder that spaceflight is NOT routine," commander Mark Polansky wrote in a Twitter update.

NASA had bumped an unmanned moon shot - its first in a decade - to give Endeavour this second chance of flying before a thermal blackout period kicks in.

The moon mission, featuring two science probes, is now scheduled for launch today. After Saturday, unfavorable sun angles will prevent Endeavour from taking off before July 11.

Cain said it was too soon to know whether NASA would be able to make the July 11 launch.