The US space agency pressed ahead Monday with preparations to pull Atlantis off the launch pad but put off a final decision on rolling the shuttle back to its massive hangar at the Kennedy Space Center on Florida's east coast. No new launch date was set.

"It's clear in our minds that we are rolling back ... unless something really extraordinary happens and we choose not to," launch director Mike Leinbach told reporters at the Kennedy Space Center in Cape Canaveral.

Officials want the shuttle to fly by September 7 on the first International Space Station (ISS) construction mission since the 2003 Columbia disaster.

Launching after that date would come in conflict with a Russian Soyuz mission. NASA officials have started negotiations with the Russians to find a solution.

If Atlantis is returned to its hangar, the shuttle will not be ready to return to the launch pad before September 7, officials said.

NASA will keep a close eye on Ernesto to see if the storm's path changes or its strength diminishes in the next hours as it crosses Cuba. But preparations were underway to have the shuttle ready to move by early Tuesday.

Officials will decide by early Tuesday whether to return Atlantis to its vehicle assembly building or let it ride the storm out on the launch pad.

"If we stay at the launch pad with the vehicle, which right now doesn't look likely, the best we can do for a launch is sometime late this weekend," Leinbach said.

Weather-related events have frustrated NASA's bid to launch on the first ISS construction mission in nearly four years.

Officials had already canceled launches on Sunday and Monday to give engineers more time to determine whether a strong lightning strike on Friday had caused any damage. The orbiter was cleared of damage late Sunday.

If it fails to fly this month, the next opportunity to launch during daylight is in late October.

NASA wants to launch during the day to take pictures of the shuttle during liftoff to check for debris from the external fuel tank that could damage the orbiter.

The Atlantis flight will be the third shuttle flight since the Columbia disaster, which was caused by debris that struck its heat shield during liftoff, dooming its return home with seven astronauts aboard.

Once it launches, Atlantis will carry six astronauts and a new 16-tonne segment with two huge solar panels for the half-finished space station.

It would be the first of 16 flights planned to complete assembly of the space station by 2010, when the three-shuttle fleet is set to retire.

The Columbia explosion forced a halt in the orbiting laboratory's construction and shifted the shuttle mission to improving safety.

After two Discovery shuttle flights in the past two years focused on safety, NASA declared it was ready to resume construction of the station, which is key to US ambitions to send humans to Mars.

The Atlantis mission is a critical first step in completing the ISS assembly.

The installation of the solar panels, which will eventually provide a quarter of the station's power, is one of the most complex parts of the assembly sequence.

Three spacewalks are planned during the 11-day mission, which will be followed by another shuttle flight planned for December for another assembly operation that officials said would be even trickier.

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