"Overnight and in the morning there was an abnormal situation - a pressure drop, an oxygen leak at the station", Roscosmos chief Dmitry Rogozin was quoted as saying by Russian news agencies.
A "minute pressure leak" was detected aboard the International Space Station Wednesday evening.
Dmitry Rogozin, head of Russian space agency Roscosmos told state news agency TASS that a "micro-fracture" was found in a side compartment of the Soyuz MS-09 spacecraft now docked with the space station.
He added: "A micro-fracture was found, most likely it is damage from the outside".
NASA is not now willing to comment on what caused the leak.
A tiny leak detected on the Space Station is now venting air into space, but NASA assures that the crew is safe and repairs are underway.
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So for now it looks like the Expedition 56 crew are probably safe, although flight controllers are continuing to monitor the pressure, just in case something does go wrong. "This is a section of the Soyuz that does not return to Earth", NASA explained. Additional updates will be posted on NASA's International Space Station blog as more information becomes available, NASA spokesperson Dan Huot told Space.com in an email. "Once the patching is complete, additional leak checks will be performed". Flight controllers are working with the crew to develop a more comprehensive long-term fix.
Six crew members are on the ISS, led by station Commander and NASA astronaut Drew Feustel.
The Soyuz MS-09 carried Sergey Prokopyev, German astronaut Alexander Gerst and NASA astronaut Serena Auñón-Chancellor into orbit June 6.
But something penetrating through the wall of the space station could also have caused the leak.