Italian astronaut Luca Parmitano on Wednesday recounted clinging to the International Space Station as he drowned in his spacesuit in the terrifying moments following an as-yet unexplained leak.

Writing on his blog from the space station, Parmitano said he felt drops of liquid on the back of his neck shortly after starting a spacewalk on July 16 -- his second foray outside the ISS.

"I feel that the temperature of the liquid is too cold to be sweat and above all I have the distinct sensation it is increasing in volume," Parmitano said in a gripping present-tense narrative of the incident in the blog post on the European Space Agency website.

Parmitano described being blinded and suffocated as he struggled to make his way back to the airlock, the water level quickly rising in his helmet.

"I don't know if I will have air or liquid in my lungs the next time I breathe.... I realise I no longer know where to go to reach the airlock," he wrote.

Parmitano said he thought about releasing a safety valve in his spacesuit to let some of the water out.

"But creating a hole in the spacesuit is really the last card to play," he remembered thinking.

"I have to get back as quickly as possible. I know that if I stay where I am, Chris (fellow astronaut Chris Cassidy) will pick me up but I do not know how much time I have left," he said.

He said communications were also becoming fainter and fainter as the water was covering his headphones and he struggled to hear instructions from Houston.

Once he was safely back inside the space station and re-pressuring, Parmitano said he began thinking that he could open up his helmet to let the water out.

"I will probably lose consciousness but it would be better than drowning in my helmet," he said.

Ultimately Parmitano did not do so and ISS teammate Karen Nyberg removed the helmet.

"Fyodor and Pavel immediately pass me a towel and I thank them without hearing what they are saying because my ears and nose are still full of water."

The US space agency NASA has launched two investigations into the incident, with engineers focusing on a possible fault in the spacesuit cooling system rather than the inner helmet drink bag.

Parmitano's spacewalk on July 16 was stopped after about an hour and a half, marking the second-shortest in the history of the International Space Station.

The 36-year-old made it safely back inside and was unhurt, though NASA experts said he faced the risk of drowning had the ordeal gone on much longer.

US spacewalks have been suspended pending the investigations.

Parmitano, who is married with two daughters and is due to return to Earth in November, has said he is ready to resume the aborted spacewalk.

"Chris and I... are ready to pick up where we left off," he said in a link-up with the ISS this month.

Parmitano concluded his blog post saying: "The skill of our engineers and the technology we have at our disposal make things that are not easy seem easy.

"It would be better not to forget that," he wrote.