"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 on Saturday to find the source of the leak.
Another Russian official played down the danger from the leak, saying it only affected the US segment of the station and the Russian segment was operating as usual.
"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.
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."
"Ammonia is used to cool the station's power channels that provide electricity to station systems," NASA said, revealing that the leak was coming from the same general area it did in a previous episode last year.
"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.
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