Washington- NASA technicians on Wednesday removed three fuel valves from the space shuttle Discovery that they feared could break off and damage the craft during launch.

The valves control the flow of hydrogen from the engines to the external fuel tank. NASA, the US space agency, planned to replace the valves with models that had flown on fewer missions, after a piece broke off from a similar valve in November on the space shuttle Endeavour. There was no damage, but the incident raised concerns that shuttles could be damaged in the future.

Since the space shuttle Columbia disintegrated on re-entry in 2003 after its heat-shielding tiles were damaged during takeoff, the US space agency has been extremely cautious about observing and preventing possible damage during liftoff.

Concern about the valves has delayed Discovery's launch several times, and a new target launch date has not yet been set. NASA officials later Wednesday announced that there would be no launch before March 12.

Discovery is to deliver the final solar panel to the orbiting International Space Station. (dpa)