The US space shuttle Endeavour soared off its seaside launch pad on Friday on a mission to upgrade the International Space Station for an expanded six-person crew.

The shuttle's twin booster rockets ignited at 2.55 a.m. yesterday, catapulting the 2.04 million kg ship with a blinding light into the clear, moonlit sky at the Kennedy Space Centre in Florida.

"It's our turn to take home improvement to a new level," shuttle commander Chris Ferguson radioed to flight controllers minutes before launch.

The launch countdown went smoothly until 15 minutes before liftoff when engineers noticed that a door on the launch pad alongside the shuttle was not properly fastened. But managers determined it would not be an issue and cleared the ship for flight.

It was NASA's first launch in nearly six months and the 124th in shuttle programme history. Just nine more launches remain before NASA is scheduled to mothball the shuttles so it can develop safer and less expensive spaceships that will return astronauts to the moon.

NASA must first complete construction of the $100 billion space station, a project of 16 partner countries. Endeavour's flight is intended to outfit the station for six full-time residents. It has been operating with half that number since assembly began a decade ago.

The shuttle carries two new sleeping compartments and a water recycling system so station crew members can purify urine and other waste water for drinking.

"We did blind taste tests of the water," said NASA's Bob Bagdigian, the system's lead engineer. "Nobody had any strong objections. Other than a faint taste of iodine, it is just as refreshing as any other kind of water."

"I've got some in my fridge," he added. "It tastes fine to me."