NASA astronauts performed an emergency spacewalk Saturday to halt an ammonia leak on the International Space Station (ISS) but it will take weeks or months to determine whether the problem has been permanently fixed.

The spacewalk was successfully completed an hour ahead of schedule at 1814 GMT, five and a half hours after flight engineers Tom Marshburn and Chris Cassidy ventured outside the ISS.

During the maneuver, 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 Chris Hadfield of Canada 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.

ISS flight director Joel Montalbano later told a press briefing the astronauts had done a "fantastic job."

Asked to put a timescale on when officials could be certain the problem had been repaired, Montalbano cautioned it could take several weeks.

"I expect it will take four weeks or five weeks, possibly longer before we have a real 100 percent characterization," he said.

"Weeks or months, it's too early to tell you. But it's going to take some time."

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

It was the 168th excursion in support of the orbiting laboratory and the fourth for both Marshburn and Cassidy.

Both US and Russian officials stressed that spacewalks are usually taxing undertakings involving ISS crews and mission control on the ground.

Hadfield, overseeing the mission, tweeted that it was a "workout" to wear a spacesuit that weighs more than 100 kilograms, according to the Russian Space Agency.

"The reason they regularly check their gloves is for damage. Even though multi-layer, even a tiny leak requires immediate haste to airlock," Hadfield said on Twitter.

The Russian space agency quoted Vladimir Solovyov, flight director for the Russian segment of the space station, as saying that "after each such sortie, guys come back like they've been through a good battle, with bruised hands and grazed shoulders."