All crew were safe and had moved to the Russian side of the facility after the leak of “harmful substances,” the Russian Federal Space Agency said on its website.
The U.S. section of the I.S.S. had been sealed off, it said.
NASA confirmed to reporters that the leak was ammonia. The space agency said the crew were safe.
Flight controllers at Mission Control in Houston saw readings that could indicate an ammonia leak in a worst case scenario and acted conservatively to protect the crew.
It is not yet known whether the alarm was triggered by a leak, a faulty sensor or a computer problem, as indicated by a recent podcast.