WASHINGTON (AFP) - 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.

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NASA astronauts on spacewalk to fix ammonia leak