A new flaw has emerged in a US-made spacesuit, forcing NASA to delay until Christmas Eve the next outing to repair the International Space Station, the space agency said Saturday.

The problem came up in the cooling unit of veteran astronaut Rick Mastracchio's spacesuit after he re-entered the space station airlock following a spacewalk that lasted 5.5 hours, NASA said.

It was not believed to be the same type of issue that caused a dangerous water leak in the helmet of Italian astronaut Luca Parmitano in July.

An investigation into that situation is ongoing.

Faced with unexpected repairs due to an equipment breakdown at the orbiting lab on December 11, NASA arranged makeshift snorkels inside the 35-year-old spacesuits and absorbent pads in the helmets for these spacewalks in case such a leak happened again.

"During repressurization of the station's airlock following the spacewalk, a spacesuit configuration issue put the suit Mastracchio was wearing in question for the next excursion -- specifically whether water entered into the suit's sublimator inside the airlock," the space agency said in a statement.

"This issue is not related to the spacesuit water leak that was seen during a July spacewalk."

A spokesman confirmed to AFP that the sublimator is the space suit's cooling unit.

Rather than allow Mastracchio, 53, to wear that suit again, astronauts are planning to work on resizing a spare spacesuit aboard the ISS for him on his next spacewalk to complete the ammonia pump module replacement.

The outing was initially set for Monday, but will now take place Tuesday, beginning at 7:10 am (1210 GMT).

NASA released the news late Saturday, after the spacewalk by the two American astronauts went faster than planned and appeared to go off without a hitch.

No live footage was broadcast, however, of the time astronauts spent inside the airlock after re-entering the space station following the spacewalk.

Astronauts made fast work of their key task for the day, disconnecting the old pump. They were also able to take on the extra task of removing the pump, which had been scheduled for Monday.

NASA mission control in Houston checked in with them frequently to see if they were experiencing any wetness in their helmets, and each time the spacewalkers reported no problems.

"Both Mastracchio and (Mike) Hopkins reported dry conditions repeatedly throughout Saturday's activities and the two were never in danger," NASA said in a statement.

Space agency officials told reporters this week that Hopkins, 44, would be wearing the suit Parmitano had worn when he experienced the leak that nearly drowned him, noting its water pump system had been replaced.

The spacewalks were called for after a faulty valve forced a partial shutdown in the system that regulates equipment temperature at the space station.

Engineers tried to fix the problem from the ground but decided after about a week that a replacement of the pump module was the best remedy.

On Saturday, Japanese astronaut Koichi Wakata operated the station's 57-foot (15-meter) robotic arm, hoisting Mastracchio and the refrigerator-sized ammonia pump module from one section of the lab to another.

With Mastracchio grasping the 780-pound (350-kilogram) module and unable to see where he was going, Wakata eased him into the right spot for stowing the pump.

The two US astronauts sported helmet cameras, and NASA TV showed the operations in real-time from the perspective of the spacewalkers.

At one point, Mastracchio said his toes were getting cold, but they warmed again once he alerted mission control in Houston and heaters were activated in his spacesuit boots.

Several hours in, when mission control asked if the pair could do even more tasks than planned for this spacewalk, possibly extending it to seven-plus hours instead of the planned six and a half, Mastracchio hesitated and then urged them to wrap up the outing.

He did not explain why, but muttered the phrase, "a couple of things," to mission control. NASA agreed and decided to end the spacewalk an hour early.