The six-hour operation by Michael Lopez-Alegria and John Herrington began at 2:50 pm (1950 GMT). The astronauts were to afix a 11,340 kilo (12.5 tonne) beam to the ISS which will form part of its future cooling system.

After securing the beam which extended the central segment from the US laboratory Destiny, the two men connected electronic cables linking it to the rest of the station.

This was the first of three scheduled space walks during Space Shuttle Endeavour's 11-day mission to the station. The shuttle's trip had a dual purpose: it brought up a replacement crew and will return the current one to Earth.

The next two space walks are scheduled for Thursday and Saturday.

Preparing for the walk, the beam -- dubbed P1 for port-side 1 -- was lifted from Endeavour's cargo hold early Tuesday by the shuttle's Canadarm1, maneuvered by Endeavour's commander, James Wetherbee.

In a delicate operation, US astronaut Peggy Whitson -- one of three astronauts aboard the ISS since June -- operated the ISS' robotic arm, Canadarm2 to maneuver the beam into place for the space walkers' work to start.

The beam will be used to extend by 14 meters (yards) the backbone of the ISS at a cost of 390 million dollars.

When complete, the ISS will measure more than 100 meters and be capable of supporting some 4,000 square meters (43,000 square feet) of solar panels that will supply the station's energy.

SPACE.WIRE