CAPE CANAVERAL, Florida (AP) _ The main oxygen generator for the international space station has failed, and the two astronauts on board will tap into an attached cargo ship's air supply this weekend, NASA said Thursday.

The space agency said the astronauts had several backup sources of oxygen and were in no immediate danger.

The generator -- a balky Russian unit that keeps breaking down -- stopped working earlier in the week. Normally, air bubbles are to blame, but this time it appeared there was a blockage in a line.

Commander Gennady Padalka is confident he can clear the blockage and get the machine running again, said NASA spokesman Rob Navias. In the meantime, Padalka took spare parts and installed them in a spare oxygen generator, which could serve as a replacement if necessary.

On Saturday, Padalka and Mike Fincke will replenish their cabin with oxygen stored aboard the docked cargo carrier, Navias said. Besides the oxygen from the cargo ship, the space station is stocked with more than a month's worth of oxygen-generating canisters.

The next cargo ship, to be launched by Russia by year's end, will deliver additional oxygen.

The space station has been short of supplies ever since NASA's shuttle fleet was grounded by the Columbia disaster last year. Padalka and Fincke's six-month mission is nearing an end. They will be replaced by another Russian and American in mid-October.