A spacewalk to work on the International Space Station ended abruptly because controllers feared the astronaut may be about to be drowned by a water-like liquid filling his helmet.

US astronaut Chris Cassidy and Italy's Luca Parmitano were less than an hour into a planned six-hour outing when Mr Parmitano reported what seemed to be water inside his helmet.

"My head is really wet and I have a feeling it's increasing," Mr Parmitano radioed to flight controllers in Houston.

Thinking it might be his drink bag leaking, Mr Parmitano drained the bag, but in the weightless environment of space, blobs of liquid continued to collect in his helmet.

"Where's it coming from?" Mr Parmitano said. "It's too much. Now it's in my eyes."

With the astronaut at risk of choking or drowning, NASA called off the spacewalk.

"It was clear that he was having trouble," flight director David Korth later told reporters on a conference call.

NASA's flight director described the experience in these terms: "Go stick your head in a fishbowl and try to walk around".

When Mr Parmitano's colleagues took off the suit inside the space station, the water floated away, and the Italian astronaut was seen blowing his nose.

Leak could have come from liquid-cooled underwear

Preliminary analysis indicates the leak did not stem from the drink bag, leading engineers to focus on the astronaut's liquid-cooled underwear as the source.

Engineers estimate about 1 to 1.5 litres of water had collected inside Mr Parmitano's spacesuit by the time station crew members were able to get to him and take off his helmet.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 42 seconds 3 m 42 s Astronaut almost drowns on spacewalk outside International Space Station ( Michael Vincent ) Download 1.7 MB

"In zero gravity, the water pools in a big glob so it doesn't necessarily go down into the suit and there's not a lot of absorptive material in the helmet," lead spacewalk officer Karina Eversley said.

When asked by a reporter if Mr Parmitano could have drowned, Ms Eversley said: "He certainly had that risk today and that's why we took it so seriously".

Expert surprised by failure, says fears of 'dire emergency' were real

Marsha Smith was a space policy expert at the Congressional Research Service for three decades and is a former director of the Space Studies Board.

She says the incident could have developed into a "very dire" emergency.

"It's always dangerous when you are outside the space station," she said.

"You need some amount of time to get back to the airlock, inside the airlock, and then to re-pressurise the airlock.

"So what they were concerned about was he could actually drown inside the helmet ... they clearly expressed concern that this could have been a very dire emergency situation. And what surprises me the most is that they are surprised at this failure and that they really don’t understand it even now.

Mr Parmitano, who last week became the first Italian to make a spacewalk, did not seem to be suffering from any ill-effects from the incident.

Analysis is continuing to determine the source of the leak and assess whether the problem is peculiar to the spacesuit Mr Parmitano wore or is part of a wider issue with the suits.

Reuters/ABC