MIAMI (AFP) - A US astronaut found water pooling in his helmet right after completing a six-plus hour spacewalk on Wednesday, raising new concerns about flaws with Nasa's spacesuits.

The water inside Terry Virts' helmet was "kind of pooling on the front side of his helmet above the eye level," European Space Agency astronaut Samantha Cristoforetti told mission control in Houston.

The water was "about three inches in diameter," she said after Virts came back into the Quest airlock and it was repressurised.

Cristoforetti said Virts was in no immediate danger, even though the amount of water had increased in the moments since he first noticed it.

Nasa television showed live images of Virts smiling inside his helmet and blowing on the water to make it ripple, before eventually removing it with Cristoforetti's help.

Virts and his fellow US astronaut Barry Wilmore had just completed a spacewalk that lasted six hours and 43 minutes.

He did not report any problems with his suit during the outing, which was aimed at preparing the space station for the arrival of more commercial spaceships in the coming years.

Nasa commentator Rob Navias said the problem was "not nearly as severe" as the near-drowning of Italian astronaut Luca Parmitano, who had to be rushed back inside the space station in 2013 after water began pooling in his helmet.

The US space agency said last week engineers were concerned about a recurring problem with the spacesuits, whereby water and condensation was building up and causing a breakdown in a part of the suit's temperature control system.

Wilmore was shipped a new suit and Virts was wearing a suit with a replacement fan pump separator that had shown some signs of corrosion ahead of the series of three spacewalks that began on Saturday.