Washington - The space shuttle Discovery blasted off for a construction mission to the International Space Station on Sunday after weeks of delays.

Discovery took off at 7:43 pm (2343 GMT) from the Kennedy Space Centre at Cape Canaveral, Florida and is to arrive at the ISS on Tuesday. The evening launch came after sunset with the flames lighting up the darkened sky.

"I've seen a lot of launches and this was the most visually beautiful launch I've ever seen. It was just beautiful," launch director Mike Leinbach said in a post-launch press conference.

The launch was to have gone off Wednesday but a leak in a hydrogen fuel vent caused it to be scrapped at the last minute. Earlier concerns about valves that control the flow of gaseous hydrogen had already delayed the mission for weeks.

The shuttle was first to have launched in February but was pushed back several times to allow NASA time to examine valves that control the flow of gaseous hydrogen from the spacecraft's engine to its external fuel tank. A valve was found to be damaged after the space shuttle Endeavour's November flight, prompting NASA to delay Discovery's mission as a precautionary measure.

There appeared to be no problems with either issue during launch, NASA officials said about an hour after lift off.

The Discovery is to deliver the fourth and final solar panel to the International Space Station on a construction mission that will enable it to double the size of its crew to six astronauts. Three spacewalks are planned to install the solar panels and conduct other construction projects.

Discovery has seven astronauts on board, including Japanese crew member Koichi Wakata, who is slated to join the permanent crew on the orbiting space station. The crew also includes two former teachers - Joseph Acaba and Richard Arnold - who will participate in spacewalks.

The 13-day mission is the first of five shuttle flights planned for this year, before the planned retirement of the fleet of ageing shuttles by the end of 2010. It has been shortened by one day and one planned spacewalk cut because of the delay.

Even with the weeks of delay for Discovery's launch, all the flights planned for this year should be able to go on as scheduled, officials said.

Discovery will also deliver a new device designed to recycle urine and sweat into drinkable water. An earlier machine had been installed on a previous mission, but astronauts never managed to get it fully functioning. The older version will be returned to Earth so engineers can examine it for flaws.