Franklin Chang-Diaz and Philippe Perrin spent more than seven hours outside the ISS replacing a wrist-joint in the faulty equipment.

On their previous two walks, the astronauts had wired up and bolted down a work platform that will allow the 17.4-metre (58-foot) arm to roam across the exterior of the outpost.

This manoeuvrability is essential to future construction work, when massive modules brought up by shuttles will need to be grasped and moved across the station to be fitted to their permanent positions.

"The arm is in operational state," Johnson Space Center spokesman John Ria-Petty said. "We have a successful completion of today's tasks."

Long stay

The shuttle Endeavour, which docked at the space station on 7 June, carried seven astronauts, including three new crew members for the station.

Expedition Five - US astronaut Peggy Whitson, and Russians Valery Korzun and Sergei Treschev - will stay on the ISS for four months.

They are replacing Expedition Four - Yury Onufrienko, Carl Walz and Daniel Busch - who will return to Earth on Endeavour on Monday.

Walz and Busch have set a new endurance mark in space for Americans during their stay on the ISS. When they land next week, they will have been in orbit for 194 days.

This is still well short of the 437 consecutive days served by the Russian cosmonaut Valery Poliakov on board the Mir space station.