The sortie was delayed by over an hour after false depressurization alarms earlier rang through the orbiting outpost and jolted mission specialists Mike Foreman and Randy Bresnik awake after just two hours of sleep, rattling preparations.

The second exterior work effort of the shuttle's 11-day mission got underway at 1431 GMT. It was shortened by 30 minutes due to the false alarms and set to last six hours.

Planning to get all of their scheduled tasks completed, the spacewalkers may still lack the necessary time to get ahead in their work, NASA said.

The pair will relocate a floating unit that measures electric charges building up on the station, install a cargo attachment system and set up a wireless video system to transmit images to the station and relay them to Earth.

Foreman and Bresnik were also due to transfer a 14,000-pound (6,350-kilogram) cargo pallet that contains spare parts to help extend the life of the station.

Mission specialist Robert Satcher aboard the ISS was managing the spacewalkers' activities and coordinating communication between them and mission control in Houston, Texas.

Bresnik, who was making his first-ever spacewalk, was awaiting the birth of his daughter back on Earth.

His wife, Rebbeca Burgin, was due to give birth to the couple's second child on Friday. If the child is born during the Atlantis mission, Bresnik would be only the second person to become a father in space.

The false depressurization and smoke detector alarms had sounded around 0253 GMT, forcing Bresnik and Foreman to exit the Quest airlock, where astronauts "camp out" before spacewalks to purge nitrogen from their bloodstream to prevent decompression sickness.

It took about an hour for everything to return back to normal on the station.

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