The Russian space agency Roscasmos said the oxygen supply system at the orbital lab in Zvezda had broken down on Wednesday. But on the American side, oxygen is working as usual.
A Roscosmos spokesman added that the safety of the crew and the International Space Station was not threatened by anything.
The issue comes at a time when three astronauts, including two Russians and an American, have arrived at the International Space Station, which now has six crew members.
This is the latest case, before the issue of air leakage in 1998.
Roscasmos said at the time that the leak was not significant and did not pose a threat to the space station's security. But it is important to look for leaks.
Staff say they now know the location and cause of the leak.
According to the staff, they have found out where the oxygen is leaking from. A spokesman for Raskasmos said he had received regular instructions from mission control on how to deal with the issue in the future.
The RIA Novosti news agency quoted Russian astronaut Gennady Padalka as saying that all parts of the Russian part of the space station had failed.
Gennady Padalka holds the world record for most days spent in space.
He added that the equipment on the space station could only be used for 15 years, as it was two decades old.
