Media playback is unsupported on your device Media caption "It is not sweat": Footage from Nasa shows the incident unfold

Nasa has aborted a spacewalk at the International Space Station (ISS) because of a dangerous water leak in an astronaut's helmet.

The leak was so bad that Luca Parmitano, Italy's first spacewalker, could not hear or speak as the spacewalk came to an abrupt end.

He asked his spacewalking partner, Christopher Cassidy, for help getting back inside the ISS.

"He looks miserable. But okay," Mr Cassidy told ground controllers.

The source of the leak wasn't immediately known but one possible source being considered was the helmet drink bag that astronauts sip from during spacewalks.

However, Mr Parmitano reported that it didn't taste like drinking water.

Before crewmates inside yanked off his helmet, Mr Parmitano said: "It's a lot of water."

Nasa rarely cuts a spacewalk short, but the Italian could have choked on the floating water droplets in the helmet.

Image caption This was the second spacewalk in eight days for Mr Parmitano (L) and Chris Cassidy (R)

The trouble cropped up barely an hour into what was to be a six-hour spacewalk to perform cabling work and other routine maintenance.

It was the astronauts' second spacewalk in eight days.

Mr Parmitano startled everyone when he announced that he felt a lot of water on the back of his head.

He had first thought the liquid was sweat brought on by the exertion of the job. But he was repeatedly assured it could not be.

Early estimates suggested about half a litre of water had leaked out.

The water eventually got into Mr Parmitano's eyes. When that happened, ground controllers ordered the two men back inside the station.

He became the first Italian to conduct a spacewalk last Tuesday, more than a month after arriving space station.

Mr Cassidy, 43, a former Navy Seal, is a veteran spacewalker midway through a six-month rotation on the ISS.