A solid fuel oxygen generating system was not working and the primary oxygen generation system, which had been shutdown to conserve power, could not be reactivated.
It is the latest in a string of mechanical breakdowns to affect the 11- year-old station but a spokeswoman for the agency said the crew were not unduly concerned.
"This is not a critical issue for them," she said.
Ed Campion, of Nasa, said that even if both systems were broken, Mir had enough oxygen to last at least a couple of days.
"If you can't get either of the two systems back up, then you're facing a serious situation," he said. "It could be that tomorrow it could be no problem, or it could be a fairly significant problem."
However, in Moscow, a mission control duty officer, denied that there was a problem at all.
"There is no oxygen problem on Mir today. On the contrary. The flight engineer reconnected the Elektron [oxygen generator] and the crew are now asleep." he said last night.
Michael Foale and his Russian crewmates have had repeated problems with the new Elektron generator, carried up by space shuttle Atlantis in May. But this is the first time since February that a crew has had serious trouble with the backup system, in which solid-fuel canisters are ignited to produce oxygen.
One of these canisters burst into flames in February, filling the station with smoke and almost causing the crew to abandon ship.
The cosmonauts were trying to ignite a canister yesterday when the system failed.
Mr Campion said the problem could be the canister itself or the device in which it is placed. But Mir had spare parts on board for the igniting device.
Russian flight controllers had ordered the crew to turn on the Elektron, which had been turned off to conserve power since last week. But it shut itself down, probably because it was overheating, Mr Campion said, leaving the crew with no means to produce oxygen.
"This is not an uncommon occurrence when an Elektron unit is reactivated after having been shut off," Nasa's status report said.
A spare Elektron device is available but has not yet been hooked up to the station's supply.Reuse content