International Space Station has a radiator leak in its power system, but no crew members are in danger and the station is operating normally, the US space agency NASA said on its website.

Crew members at the orbital outpost spotted white flakes of ammonia floating away from the space station at about 1330 GMT on Thursday, NASA said, and fixing the leak might require that a portion of the station's cooling system be shut down for about 48 hours.

"The station continues to operate normally otherwise and the crew is in no danger," it said.

In an audio exchange posted on the agency's website, Commander Chris Hadfield, who is Canadian, said he could see "a very steady stream of flakes or bits" coming from the area of one of several cooling loops.

Officials said the leak appeared to be getting worse.

The ammonia flakes were seen floating away from an area of the space station's P6 truss structure, the agency said. It was not clear whether it was related to a previous leak in late 2012.

Ammonia is used to cool the equipment that provides power to the station's systems, NASA said. Each array of solar battery cells has its own cooling loop.

The space station, which is staffed by rotating crews of six astronauts and cosmonauts, is a $100 billion research outpost owned by the United States and Russia in partnership with Europe, Japan and Canada.