NASA astronauts performed an emergency spacewalk Saturday to halt an ammonia leak on the International Space Station 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."

"We're very happy, we didn't see any sign of leaks but it's going to take weeks to see if we did in fact stop the leak," Montalbano said.

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, who have worked together before.

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 220 pounds (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."

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."

The US space agency said ammonia was leaking from the same general area as in a previous episode in November last year.

A meteorite or a piece of orbital debris is suspected to have hit the cooling radiator and caused the problem, which ISS program manager Michael Suffredini described as an "annoyance because of all the work we have to do to work around the problem."

The issue took a turn for the worse on Thursday, when it began leaking about five pounds of ammonia per day, compared with a previous level of five pounds per year.

Hadfield earlier described the leak as 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 said in a recording of the conversation posted by NASA.

Hadfield, Marshburn and Russian cosmonaut Roman Romanenko are set to return to Earth early Tuesday after completing their half-year stint aboard the station. And Saturday's spacewalk is not expected to interfere with their planned departure.

Cassidy is also set to perform two scheduled spacewalks in July, NASA said.