NASA discovered a gouge on Endeavour's belly few days after it headed for the international space station to perform assembly work, repairs and deliver supplies.

The failure has possibly been caused by ice that broke off the fuel tank a minute after lift off.

The gouge was spotted in a photo taken by the space station crew shortly before Endeavour delivered teacher-astronaut Barbara Morgan and her six crewmates to the orbiting outpost.

It was first said that the debris, which fell off during the launch, did not have enough velocity to do any damage.

"What does this mean? I don't know at this point," said John Shannon, chairman of the mission management team.

"If the gouge is deep enough, the shuttle astronauts may have to patch it during a spacewalk," he added.

NASA has been extremely watchful for any damage to a shuttle's external tank or heat shield after the 2003 Columbia disaster.

Space shuttle Endeavour seamlessly lifted off into the blue skies above Florida at 6:36 p.m on Wednesday with a seven-member crew on board.

The Endeavour crew - six Americans and one Canadian - includes Barbara Morgan, a former Idaho teacher who was selected as teacher Christa McAuliffe's backup for the doomed Challenger mission 21 years ago.

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