National Aeronautics and Space Administration officials said the mission entails the most complex work ever undertaken at the nearly eight-year-old, half-finished ISS.

Completing the space station is central to US ambitions to fly humans to Mars.

"What you saw today is a flawless count, a majestic launch. This vehicle has not flown since 2001 and not everything in the count leading up to this day was easy," said NASA administrator Michael Griffin.

The shuttle reached orbit about nine minutes after liftoff, carrying six astronauts and cargo containing two huge solar panels to generate more power for the ISS.

Pieces of foam that broke off the shuttle's external tank during the liftoff did not initially appear to pose a significant risk, the US space agency said.

Launch coordinator Leroy Cain said that the foam broke off probably too late into the launch to have done any serious damage.

"The earliest indication that we have of any event in the way of shedding debris was after four minutes, seven seconds ... well into the ascent phase where we don't really have a debris concern, since there is no dynamic pressure and there is therefore no way for the debris to create a critical impact," Cain said.

He said NASA officials reviewed film of the launch and "we didn't see anything in the region of highest concern," but added: "It's extremely preliminary" to rule out any significant damage.

Foam shedding during launch has plagued NASA since the space shuttle Columbia disintegrated in February 2003 over Texas due to debris-damaged insulation, killing the seven astronauts aboard.

The debris problem has continued to affect the past several shuttle missions and contributed to the long delay in undertaking missions for ISS construction.

Atlantis's two rocket boosters successfully separated from the orbiter two minutes after blasting off from the Kennedy Space Center in Cape Canaveral, Florida, and the shuttle later jettisoned its massive external fuel tank.

After settling into orbit 225 kilometers (140 miles) above Earth, the shuttle lit two small motors designed to help maneuver it into a path that will take it to a 350 kilometer- (218 mile-) high orbit and the ISS, where it is expected to dock Monday at 6:46 am (1046 GMT).

Kyle Herring, the NASA spokesman for the flight, announced all systems were functioning normally after the launch.

Atlantis initially was supposed to launch on August 27, but the mission was delayed five times due to a lightning strike, a looming tropical storm and technical glitches.

Friday's launch was put off just one hour before liftoff due to a faulty fuel sensor. The sensor appeared to work again after the fuel tank was emptied and refilled.

Saturday's liftoff began the third shuttle flight since the 2003 Columbia disaster. After space shuttle Discovery returned in July from a second mission aimed at making flights safer, NASA declared that it was ready to resume construction of the half-finished ISS.

Beginning with the Atlantis mission, NASA plans 16 shuttle trips to complete the orbiting laboratory by 2010, when the three-shuttle fleet is to be retired. The last ISS construction mission was in November 2002.

Atlantis is transporting a 16-tonne segment with two huge solar panels that will double the space station's ability to produce power from sunlight and ultimately provide a quarter of the power for the completed ISS.

Three lengthy spacewalks are planned to install the solar panels.

The Atlantis crew is commanded by Brent Jett and co-piloted by Chris Ferguson. The four mission specialists are Daniel Burbank, Heide Stefanyshyn-Piper, Joe Tanner and Canadian Steve MacLean of the Canadian Space Agency.

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