The leak, discovered late last month, sparked a fresh row over the 16-nation station between the United States and Russia. But both acknowledged that it posed no danger to the two-man crew.
"The leak is in a flex hose in the lab window," Jim Newman, director of NASA's Human Space Flight Programme in Russia, told Reuters by telephone.
"It's ninety-five percent at this stage... It is most likely the culprit."
The problem, if confirmed, would probably be solved by replacing the hose which helps keep air and condensation out of the windows around the laboratory, he said.
Newman believed there was no longer any need to proceed with the plan to isolate the crewmen - US astronaut Michael Foale and Russian cosmonaut Alexander Kaleri - in a station module for five days.
That plan called for closing hatches connecting the four main modules - two Russian and two American - in order to trace the leak.
NASA officials said last week they believed the orbital platform was continuing to lose air pressure, while Russian officials said the pressure had stabilised at a normal level.
Previous squabbles between the two space powers have included US safety concerns before the current crew's launch and Russia's belief that the United States should contribute more funds to the project.