A piece of ice struck the shuttle shortly after Wednesday's liftoff from Cape Canaveral, Florida, leaving what appears to be a three square inch (19 square centimetre) gouge near the hatch of one of the shuttle's landing gears, mission manager John Shannon said.

Small white marks were also visible on other thermal tiles surrounding the gouged area, he told a news conference.

He said Nasa was trying to determine the extent of the apparent damage, adding: "What this means, I don't know at this point."

The possible damage was detected Friday after ISS crew members took 296 pictures of the shuttle's underside while it performed a backflip during its approach to the station. The pictures were analysed by Nasa experts on Earth.

Astronauts on Sunday will use a camera attached to a robotic arm to closely inspect the area of concern, and a laser to determine exactly the depth of the gouge, Shannon said.

If repairs are deemed necessary, he added, the Endeavour mission would be extended by an additional space walk. He said that materials to patch up the thermal shield were available to the astronauts.