"A micro fracture was found, most likely it is damage from the outside".
In a statement, NASA added: "The leak, which was detected Wednesday night by flight controllers as the Expedition 56 crew slept, resulted in a small loss of cabin pressure".
He said the fracture was found on the Soyuz ship that brought astronauts to the ISS in June for a six-month mission and is now docked with the space station.
The leak was discovered at around 7 p.m., (EDT) Wednesday, by flight controllers in Houston and Moscow.
The six astronauts were alerted to the breach on Thursday morning and fix procedures are now in effect.
NASA confirmed the problem, saying it consisted of a "minute pressure leak" and that the crew was repairing it.
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Once the hole was identified, crewmembers applied Kapton tape, which slowed the leak.
NASA and the Russian space agency Roscosmos operate the orbiting space lab along with the European Space Agency, Japan's JAXA and the Canadian Space Agency. When the crew awoke this morning, mission controllers both in Houston and outside of Moscow attempted to pinpoint the leak.
The astronauts now onboard the ISS are NASA astronuats Drew Feustel, Serena Auñón-Chancellor and Ricky Arnold, ESA astronaut Alexander Gerst and Russian astronauts Sergei Prokopyev and Oleg Artemyev.
NASA said the crew were never in danger. The space station is heated and pressurized with an air mixture so that the astronauts can breathe normally, and it requires an airtight seal to ensure that the air can be continuously recycled.