Two US astronauts have embarked on a six-and-a-half hour spacewalk to install a special parking spot at the International Space Station.

The spacewalk began around 1200 GMT, when Americans Jeff Williams and Kate Rubins switched their spacesuits to internal battery power. Shortly afterward, they made their way outside the orbiting laboratory to begin the work of attaching an international docking adaptor.

It was launched aboard SpaceX's, "Dragon" cargo spacecraft last month. The docking adaptor will be the first of two such additions to the ISS. The second is expected to be installed in 2018.