With more private spaceship traffic expected at the International Space Station in the coming years, two spacewalking US astronauts installed a special parking spot for them on Friday.

Americans Jeff Williams and Kate Rubins floated outside the orbiting laboratory for a spacewalk lasting five hours and 58 minutes to attach the first of two international docking adaptors.

The astronauts spent more than two hours tying down the adaptor, after which robotic machinery at the space station completed the hard mate, making the attachment permanent.

"With that, we have a new port of call," NASA commentator Rob Navias said as the space station flew over Singapore at 10:40 am (1440 GMT).

During the rest of the spacewalk, astronauts connected power and data cables for the adaptor.

The fittings will enable the space station to share power and data with visiting spaceships.

The spacewalk was the fourth for Williams, a veteran astronaut who on Wednesday will surpass US astronaut Scott Kelly's record for the most cumulative days in space for an American.

Kelly has 520 days in space over his career.

Williams will have 534 days in space by the time he wraps up his stint at the ISS and returns to Earth in early September.

The spacewalk was Rubins's first. She is the 12th woman to walk in space.

- 'Gateway to future' -

NASA describes the docking adaptor as a "metaphorical gateway to a future" that will allow a new generation of US spacecraft -- the first since the space shuttle program ended in 2011 -- to carry astronauts to the space station.

The second docking adaptor is expected to be launched in late 2017, Navias said.

ISS operations integration manager Kenneth Todd called Friday's installation a "very significant milestone on the path to establishing commercial crew capability."

Built by Boeing, the circular adaptor measures around 42 inches (one meter) tall and about 63 inches wide.

The adaptors will work with Boeing's CST-100 Starliner and SpaceX's Crew Dragon, two spaceships under construction that are planned to ferry astronauts to the space station.

The docking adaptor is more sophisticated than past equipment because it will allow automatic parking instead of the current grapple and berthing process, which is managed by astronauts.

- Spacewalk Sept 1 -

During the last US spacewalk on January 15, a problem with American Tim Kopra's spacesuit allowed a small amount of water to build up inside his helmet by the end of the outing.

It was the latest in a series of spacesuit issues, but not as severe as an emergency in 2013 when Italian astronaut Luca Parmitano's helmet flooded, forcing him to end his spacewalk early.

Williams experienced a problem with an earpiece in his suit near the end of Friday's outing, briefly interfering with his ability to hear mission control, but NASA said the issue was not serious.

The US space agency is planning another spacewalk on September 1 to retract one of the thermal radiators outside the space station.

Astronauts unsuccessfully tried to push it back into position last year.