NASA astronauts are performing an emergency spacewalk to try to stop ammonia from leaking from the International Space Station's power system, NASA television shows.

During the spacewalk, expected to last six and a half hours, flight engineers Tom Marshburn and Chris Cassidy will inspect and possibly repair the ammonia leak that affected the US segment of the orbiting laboratory on Thursday.

'Spacewalk under way to repair ammonia leak,' said the headline of a US space agency statement. Ammonia is used to cool the station's power system.

However the astronauts did not detect any signs of ammonia leaking into space in the first hour of their mission, NASA television said, adding that they 'have reported nothing out of the ordinary so far.'

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

It will be the 168th excursion in support of the orbiting laboratory and the fourth for both Marshburn and Cassidy, who have worked together before.

ISS commander Chris Hadfield and Russian cosmonaut Pavel Vinogradov helped Marshburn and Cassidy suit up for the excursion, inspecting their gear and harnesses and making a series of elaborate checks.

'Good Morning, Earth! A complex vital day on Space Station,' Hadfield tweeted before the spacewalk began.

'Busy day!'

NASA has stressed that the lives of the multinational crew were not in danger, but both Russian and US space experts called the leak 'serious'.

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NASA astronauts on spacewalk to fix leak