"It looks it was an ice impact to me," mission manager John Shannon told a news conference after the shuttle docked with the International Space Station (ISS) for an 11-day mission.

Shannon added that NASA was trying to determine the extent of the potential damage. "What this means, I don't know at this point."

An analysis of images taken during Wednesday's liftoff in Cape Canaveral, Florida, shows that the shuttle's underbelly was hit by a piece of ice, rather than foam, from its external fuel tank as previously believed, he said.

The impact left what appears to be a three square inch (19 square centimeter) gouge near the hatch of one of the shuttle's landing gears.

The possible damage was detected Friday after ISS crew members photographed the shuttle's underside while it performed a backflip during its approach to the station.

Astronauts on Sunday will use a camera attached to a robotic arm to closely inspect the area of concern, Shannon said.

The US space agency has carefully inspected the orbiter's protective thermal tiles in the missions that followed the shuttle Columbia disaster of February 2003.

Columbia's heat shield was pierced by a piece of insulating foam that peeled off its external fuel tank during liftoff, causing the shuttle to disintegrate into a ball of fire as it re-entered Earth's atmosphere. Seven astronauts died.

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