WASHINGTON (AFP) – Engineers have found two cracks on the external tank of the space shuttle Discovery after delaying its launch until the end of this month following a hydrogen leak, NASA said Thursday.

The cracks were found late Wednesday on an aluminum strip separating the liquid oxygen and liquid hydrogen tanks, after technicians removed a segment of foam insulation that developed a 51-centimeter (20-inch) fissure during the November 5 launch attempt.

The strip, known as a stringer, provides structural integrity for the external tank where liquid hydrogen and oxygen are stored. Combined at low temperatures, they provide the powerful fuel to blast the shuttle through space.

The cracks measure around 23 centimeters (nine inches) long and have increased doubts whether Discovery will be able to launch on its last ever flight to the International Space Station this year as planned.

"Engineers are reviewing images of the cracks to determine the best possible repair method," the statement said.

Such a repair has never been made with the space shuttle on the launch pad.

If the teams can repair the cracks, NASA has set the next launch attempt for no earlier than November 30 at 4:05 am EST (0905 GMT), but said that could change slightly as the date approaches.

To get in a flight this year, Discovery must blast off for the ISS before December 6. Otherwise it will have to wait until February, the same month that the last ever shuttle launch is scheduled before the fleet is mothballed for good.

Engineers were to meet Friday to discuss the status of the repairs, including replacing a ground umbilical cord and removing and replacing a cockpit panel.

Discovery's 11-day mission with its all-American crew of six is to deliver a pressurized logistics module called Leonardo to the ISS, which will be permanently attached to the space station to provide more storage space.

The shuttle will also bring Robonaut 2, the first human-like robot in space and a permanent addition to the orbiting space station, as well as spare parts.

Two space walks, for maintenance work and component installation, are scheduled.