The tank, which is a key part of the cooling system on the ISS, was installed Tuesday during what was supposed to be the last space walk by US shuttle astronauts.

But a faulty nitrogen valve on the pressurizer for the ammonia has prevented the new cooling system from working, said Dan Huot, a spokesman for NASA in Houston.

"Right now, ground controllers are going through their procedures," Huot said.

"They have been doing a couple of calling procedures to try to get the valve to respond, but it is not responding. They are trying to figure it out."

A fourth space walk could be called for to fix the valve, said Huot.

"That's one potential way, because they do have a spare box with a nitrogen pump in it" at the ISS, he said.

Astronauts Rick Mastracchio and Clayton Anderson installed the ammonia tank during a six-and-a-half hour space walk on Tuesday, which was supposed to be the final outing for shuttle astronauts before they return to Earth on board Discovery on Monday.

The space walk took longer than originally scheduled because the astronauts had problems stowing the empty ammonia tank assembly on a carrier in the shuttle's cargo bay, to bring it back to Earth.

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