Two American astronauts have replaced a large pump controller box outside the international space station that may be the source of an ammonia leak in the orbiting laboratory's power cooling system.

NASA says astronauts Chris Cassidy and Tom Marshburn removed the 113-kilogram box and replaced it with a spare about two-and-a-half hours into their spacewalk.

On Friday, the space station's Canadian commander, Chris Hadfield, described the ammonia leak as a "serious situation," but one that had been stabilized by the six-member crew.

The U.S. space agency NASA said the crew was not in danger and that the station otherwise continues to operate normally.

Space station crew members reported the leak Thursday.

Hadfield described the problem to the spaceship's ground controllers at NASA's command center in Houston, Texas.

"Houston - Station. This is Chris. Space to Ground One.... I was back in the docking compartment looking. I was just talking with Tom and Chris who sounds like they can back up completely what they saw, a very steady stream of flakes or bits coming out of... then rotating towards the... as the truss is rotating, so... we're just waiting to rotate now in order to get a decent view out of the gym.''

Ammonia is used to cool the station's power systems that provide electricity to the station.

The station had a similar leak last year, but it is not clear whether this leak is from the same source.