An Italian astronaut discovered "funny" tasting water leaking in his helmet during a spacewalk at the International Space Station Tuesday, forcing an early end to the outing, NASA said.

The US space agency said it was probing what caused the situation, but stressed that the astronauts were not in any danger during what was supposed to be a routine repair job at the orbiting lab.

Instead, it became the second shortest spacewalk in history, following a 14-minute outing in 2004 that was terminated early due to a pressure sensor failure in the Russian-made suits, a NASA spokesman said.

Just over an hour into Tuesday's walk, Luca Parmitano of the European Space Agency reported fluid inside his helmet and was rushed back into the space station Quest airlock.

As crew members helped remove his headgear, globs of water could be seen floating away.

Shortly afterward, he described for his fellow astronauts a strange liquid that appeared to come from inside the back of his helmet.

"Luca says the water tastes really funny," US astronaut and fellow spacewalker Chris Cassidy told NASA mission control in Houston.

"To him the water clearly did not taste like our normal drinking water," said Cassidy, suggesting that engineers investigate the fluid that fills up the suits ahead of the spacewalk for clues.

Parmitano's long underwear inside the suit was dry around his midsection, and it appeared the leak had come from the vent port near the back of Parmitano's helmet, Cassidy added.

Cassidy said his colleague was doing "okay," and NASA TV showed images of Parmitano floating inside the space station and blowing his nose with a tissue.

"Neither astronaut was in serious danger throughout," a NASA commentator said, but added the cause of the leak was "not easily identifiable."

Parmitano, 36, is the first Italian to walk in space, and Tuesday's outing was the second of his career.

The purpose of the spacewalk, the second of two planned this month, was to prepare the ISS for a new Russian module and to make some repairs.

The outing lasted one hour 32 minutes, NASA said. It was supposed to last six hours and 15 minutes.

Parmitano blogged about his first experience suiting up in the heavy gear and walking in space just last week, when he stepped out of the ISS on July 9 with Cassidy.

"Putting on the space-suit requires considerable physical effort, especially for the top part," he wrote of that first sortie on blogs.esa.int/luca-parmitano.

"The torso section is stiff and tight, especially for me because I chose a smaller size in order to be more agile," he added.

"And I will be more agile but the price I pay is having to contort my arms and shoulders while pushing with my feet as hard as I can, until my head fits through the metal collar and my hands pass through the wristbands of the suit."

Soon after, he described the elation of seeing the Earth spin beneath him for the first time.

"I feel so good I can't stop myself from smiling," he said.

Parmitano arrived at the ISS on May 28 for a six-month mission, joining Cassidy, Russian commander Pavel Vinogradov and flight engineer Alexander Misurkin who were already there.

The Italian arrived aboard a Soyuz with US astronaut Karen Nyberg and Russian cosmonaut Fyodor Yurchikhin.

Canadian astronaut Chris Hadfield, who recently returned to Earth after a stint at the ISS, tweeted that the spacewalk was "hard risky work."

"While outside, lots of water inexplicably collected in Luca's helmet. Where is it coming from? To be safe, NASA stopped the spacewalk. Good idea," Hadfield said.

NASA said it would brief reporters from Houston no earlier than 3:30 pm Central Time (2030 GMT).