The International Space Station crew Friday were preparing for an emergency spacewalk to fix a "very serious' leak of ammonia from the orbiting laboratory's power system seeping into space.

NASA emphasised that the lives of the multinational crew were not in danger but both Russian and US space experts were scrambling to swiftly fix the problem.

ISS commander Chris Hadfield of Canada in a dramatic exchange with mission control said the crew were witnessing a "very steady stream of flakes or bits" of ammonia drifting into space.

The flakes were moving "evenly and repeatedly enough that it looks like they were coming from a point source," he added in a recording of the conversation posted by NASA.

Hadfield later tweeted from the space station that the leak was a "serious situation" but had "been stabilised" thanks to efforts by the crew.

"Indeed, they have a serious defect, very serious," Vladimir Solovyov, flight director for the Russian segment of the space station, was quoted as saying by the Interfax news agency.

NASA said the leak of ammonia, which is used to cool the station's power system, did not pose any danger to the crew of six astronauts on board.

The US space agency said on its website that while the rate of ammonia leaking from the station's truss structure had increased, the "station continues to operate normally otherwise and the crew is in no danger."

NASA revealed that the leak was coming from the same general area as in a previous episode in November last year.

It said early analysis by specialists indicated that the leak rate could result "in a shutdown of this one cooling loop in about 48 hours."

Hadfield indicated on Twitter that US astronauts Tom Marshburn and Chris Cassidy are likely to step into open space on Saturday to find the source of the leak.

"Big change in plans, spacewalk tomorrow (Saturday), Chris Cassidy and Tom Marshburn are getting suits and airlock ready. Cool!" Hadfield tweeted.

He added: "The whole team is ticking like clockwork, readying for tomorrow. I am so proud to be Commander of this crew. Such great, capable, fun people."

The Interfax news agency quoted a space industry source as saying that Mashburn and Cassidy were set to make a six-hour walk starting around 1200 GMT Saturday.

Another Russian official however played down the danger from the leak, saying it only affected the US segment of the station.

"This is not critical," the state RIA Novosti news agency quoted Alexei Krasnov, head of manned flight programmes at the Russian Space Agency, as saying.

"It's not the first time such a situation has happened, unfortunately," Krasnov said.

There has been no official statement from the Russian Space Agency.

The space news website Spaceflight101 called the leak "major."

Hadfield, Marshburn and Russian cosmonaut Roman Romanenko are set to return to Earth early on May 14 after completing their half-year stint aboard the station.

It is not clear if the leak could affect their plans on Tuesday.

Since 2009 there have been teams of six astronauts and cosmonauts aboard the space station, whose capacity was previously limited to only three people.

Russia has suffered several recent setbacks in its space programme, notably losing expensive satellites and an unmanned supply ship to the ISS but the manned missions have been flawless.