Oxygen and food will be worrisomely low on the international space station by Christmas, and it is crucial that a Russian supply ship get there by then, a top NASA official said Friday.

Complicating matters is the breakdown of the station's primary oxygen generator. The unit has barely worked this month despite intensive repair efforts by the two astronauts on board, and the men have had to tap into backup oxygen supplies.

The next Russian cargo ship is scheduled to be launched Dec. 23. That is around the time that supplies -- notably oxygen and food -- will be getting tight, said Bill Gerstenmaier, the space station program manager.

The situation will not be nearly as dire if the astronauts manage to get their main oxygen generator working again. But if they do not and a cargo ship cannot be launched by year's end, the space station will have to be abandoned, with the crew returning to Earth in their docked Russian capsule.

"We're a long way away from de-manning," Gerstenmaier said.

Gerstenmaier said the current crewmen -- Russian Gennady Padalka and American Edward "Mike" Fincke -- are in no immediate danger because of the multiple oxygen reserves on board. He said the same will be true for their replacements, who will rocket away from Kazakhstan on Oct. 11 on a six-month mission. Their capsule will carry more spare parts for the oxygen generator but have room for little else.

"When you couple all of those things together, you take a look at that and it's worth it to go ahead and put the crew on orbit," Gerstenmaier said. "They're not at any undue risk. There's plenty of capability. We have plenty of time to react, and we've got a lot of time to really work this issue to see what happens and what comes."

The space agency would not want to keep a crew on board if oxygen and food dwindled to less than about a 45-day supply, Gerstenmaier said. That "red line" would be reached right around the time the next supply ship is to arrive, on Christmas Day.

NASA has had to rely on the Russian Space Agency for all station crew and cargo deliveries since last year's Columbia accident. The shuttle fleet is not expected to resume flying until next spring at the earliest.

The Russian-built oxygen generator -- a source of repeated trouble -- shut down more than two weeks ago. Padalka has replaced suspect parts and flushed out clogged lines to little avail.

Engineers' latest theory is that the valve used for venting hydrogen overboard may be contaminated. The machine produces oxygen by breaking down waste water into its two basic chemical elements. Another oxygen generator is on board and might be usable, but it has its own history of malfunctions, Gerstenmaier said.

A cargo ship to service the international space station, above, hampered by a broken main generator, is set for launch Dec. 23.