"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 micro fracture was found, most likely it is damage from the outside. The design engineers believe it is the result of a micrometeorite," he said.

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 currently docked with the space station. The fracture will be patched from the inside, he said.

NASA confirmed the problem, saying it consisted of a "minute pressure leak" and that the crew was repairing it.

Six men are currently orbiting Earth aboard the ISS, including NASA astronauts Drew Feustel, Ricky Arnold and Serena Aunon, as well as Alexander Gerst of the European Space Agency and two Russian cosmonauts -- Oleg Artemyev and Sergei Prokopyev.