Two NASA astronauts wrapped up a successful spacewalk Thursday to repair the International Space Station's aging robotic arm, the US space agency said.

The outing by Americans Randy Bresnik and Mark Vande Hei ended at 3 pm (1900 GMT), marking a "very successful day," a NASA spokesman said.

The spacewalk lasted six hours and 55 minutes, almost a half hour longer than planned because the pair managed to tack on a few extra jobs that had been planned for next week.

Their main work involved the latching end of the Canadian-made arm, known as Canadarm2.

They replaced one of two Latching End Effectors (LEE) which had lost the ability to grip effectively, said the US space agency.

The 57.7 foot-long (18 meter) arm was instrumental in assembling the space station and is used to reach out and grab approaching cargo ships.

The robotic arm has been a key piece of equipment at the orbiting outpost for more than 16 years, but began malfunctioning in August.

NASA wants to restore its full capability before the next US cargo ship arrives next month, carrying supplies for the six astronauts living in orbit.

Thursday's spacewalk was the first of three scheduled spacewalks this month aimed at repairing and maintaining various pieces of equipment outside the ISS, and was the 203rd spacewalk in the history of the space station.

Vande Hei and Bresnik plan to step out on another spacewalk October 10, with the third set for October 18.

"The second and third spacewalks will be devoted to lubricating the newly installed end effector and replacing cameras on the left side of the station's truss and the right side of the station's US Destiny laboratory," NASA said.