"Indeed, they have a serious defect, very serious," Vladimir Solovyov, flight director for the Russian segment of the space station was quoted as saying by the Interfax news agency.
The US space agency NASA earlier said the ammonia leak detected in the cooling system did not pose a danger to astronauts on board the station.
Solovyov said officials were considering whether to send ISS crew members into open space to fix the leak.
Interfax, citing a space industry source, said NASA astronauts Tom Mashburn and Chris Cassidy are likely to step into open space tomorrow to find the source of the leak.
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"This is not critical," the state RIA Novosti news agency quoted Alexei Krasnov, head of manned flight programs at the Russian Space Agency, as saying.
"It's not the first time such a situation happens, unfortunately."
There was no official statement from the Russian Space Agency.
NASA said crew members had spotted small white flakes floating away from an area outside the craft before reporting the incident.
The space agency said on its website that while the rate of ammonia leaking from the station's truss structure had increased, the "station continues to operate normally otherwise and the crew is in no danger."
"This ammonia loop is the same one that spacewalkers attempted to troubleshoot a leak on during a spacewalk on Nov. 1, 2012," the statement said.
"It is not yet known whether this increased ammonia flow is from the same leak, which at the time, was not visible."
The ISS is currently crewed by six staff, under the command of Canadian Chris Hadfield.