Mission specialists Jerry Ross and Jim Newman left the shuttle Endeavour's airlock and began their walk shortly before 2230 (GMT)/1730 (EST).

The two astronauts are expected to take about six-and-a-half hours to connect cables carrying electrical power and avionics between the Russian-built Zarya station and the US-built Unity.

Lead Flight Director Bob Castle, at Mission Control in Houston, said the "nervous system, if not the heart of the station, will be coming alive" when ground controllers signal the two modules to power up.

US astronauts Jerry Ross and Jim Newman at work

Most of the cables on the International Space Station will run along the outside of the structure, avoiding the chaotic tangle that ran through the corridors of the Russian Mir station and hindered hatch closings in emergencies.

Spacewalk programme

In a pre-flight interview, Ross said: "The first EVA (extra vehicular activity) is by far the most critical one.

"It's the one that is required to hook together the elements of the station that are up there and to permit us to start activating the US-built parts of that station."

Ross and Newman have two other spacewalks planned for the 12-day mission. As they reach the top of the Zarya power station they will be working about seven stories above Endeavour.

The two modules have been successfully joined

Falling is not a problem in weightlessness, but working such a distance from the safe haven of the shuttle could be.

Frank Culbertson, Operations Manager for the space station programme, said: "They may be the most difficult EVAs ever performed. But they will get even harder."

More than 1,100 hours of spacewalks will be required during the construction phase of the station's 10- to 15-year life - more than in the entire history of human space flight.

The space station became a reality on Sunday as the first two components were brought together in orbit by Endeavour's crew.

In time, more than 100 space station components will be assembled in orbit in one of the most ambitious and expensive engineering feats ever undertaken.

Sixteen nations are involved in the $60bn project, which will require more than 40 manned missions and hundreds of hours of high-risk space walks during the construction phase.

Endeavour's six-member crew will enter the station on Thursday, finding it much like the unfinished interior of a new home.