The six-member crew on Thursday noticed white flakes of ammonia leaking out of the station.
Ammonia runs through multiple radiator loops to cool the station's power system.
NASA said the leak is increasing from one previously leaking loop that can be bypassed if needed.
NASA spokesman Bob Jacobs said engineers are working on rerouting electronics just in case the loop shuts down. The Earth-orbiting station has backup systems.
Space station Commander Chris Hadfield of Canada tweeted that the problem, while serious, was stabilized. Officials will know more Friday.
The space station always has enough emergency escape ships for the crew, but there are no plans to use them.