NASA astronauts have replaced a pump during an emergency spacewalk to stop an ammonia leak at the International Space Station's power system, NASA television shows.

The spacewalk was successfully completed an hour ahead of schedule at 1814 GMT (0414 AEST Sunday) five and a half hours after flight engineers Tom Marshburn and Chris Cassidy ventured outside the space station.

During the manoeuvre, Marshburn and Cassidy attached a spare pump and flow control sub assembly box to replace the one suspected to be the source of an ammonia leak that affected the US segment of the orbiting laboratory on Thursday.

Ammonia is used to cool the station's power system.

The new pump was turned on about an hour later, and after about 30 minutes of careful observation by the astronauts and mission control, ISS commander Canadian Chris Hadfield tweeted: 'No leaks! We're bringing Tom Chris back inside.'

Although the pump will continue to be observed through instruments, the early indications were positive that 'we have climbed a big mountain on solving the ammonia leak,' a commentator at mission control said on NASA television.

But he cautioned that it will take several months of investigation and observation before it can be confirmed that the pump was 'the smoking gun' that caused the leak.

Before re-entering the space station, the astronauts took precautions against ammonia contamination, despite not seeing any evidence of it in the area around the space station.

That included an hour-long 'bake-off' to allow the sun to burn off any ammonia traces on their space suits.

Officials said the emergency spacewalk set a precedent because it was conducted at such short notice.

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NASA astronauts finish spacewalk