Washington, Aug 29 (DPA) The space shuttle Discovery was set for a midnight launch on Friday after problems associated with a fuelling valve that forced previous delays were resolved, NASA officials said.

Discovery is set to blast off at 11.59 p.m. (0359 GMT Saturday) from Cape Canaveral, Florida, en route to the International Space Station (ISS).

NASA was fuelling the shuttle for the launch and the readings on what had been a suspect valve were normal. But weather was beginning to emerge as a potential challenge.

Forecasts showed there was a 60 percent chance of acceptable conditions and meteorologists were monitoring clouds and thunderstorms within 30 km of the facility.

'The weather remains the primary concern,' NASA said.

Weather forced a delay of a planned takeoff on Tuesday and then problems with the valve scrubbed a scheduled liftoff Wednesday. NASA then sought to fire up Discovery early Friday, but elected to postpone it again to review data in connection to the valve.