US astronauts Christina Koch and Jessica Mir made history yesterday when they emerged from the International Space Station in the first fully spacewalk.

NASA's long-awaited event took place during a relatively routine mission to replace broken batteries on the outer surface of the station.

NASA said in a live video of the operation that Koch and Mir left white clothes from the 408-kilometer-high station to outer space to replace a damaged power unit designed to help adapt the energy stored from the station's solar panels.

The operation lasted about five hours and comes after a first attempt at the first full women's spacewalk in March. The attempt was canceled because the space suit had not been prepared for one of the astronauts.

US President Donald Trump congratulated the two women in a video call and thanked them for their courage.

"This is truly historic," he told them from the White House conference room in the presence of his deputy Mike Pence, his daughter, his adviser Ivanka Trump and NASA director Jim Pridenstein.