Two astronauts from the US Space Shuttle Endeavour have made history by becoming the first people to step inside the International Space Station.

US commander Robert Cabana and Russian cosmonaut Sergei Krikalev excitedly unlocked three hatches before floating side by side into Unity, the US-made science outpost.

They carried lanterns to see inside the 36-foot (10 metre) cylindrical chamber.

Lights on

Once inside the space station, the astronauts marked the moment by simply switching on the lights.

Commander Cabana paves the way forward

They shared the moment with the world as they carried a camera into the space station.

"It's fantastic. I can't say how much this means to all of us" said Robert Cabana.

"We remember when Unity was just an aluminium shell. It's really nice to be in a new home."

Mission Control replied: "You've opened the doors to a whole new era in spaceflight."

Endeavour's six-member crew began assembling the first two elements of the $60bn outpost on Sunday, but all of the work was outside the seven-level space station prior to the hatch opening.

The space station consists of only two rooms so far and is still more than five years from completion.

Astronauts work to install a communications system to Unity

The crew is now carrying out interior work, including the installation of a communication system to provide a link between the shuttle and the space station when the shuttle is not docked.

More than 100 space station components will eventually be assembled in orbit in one of the most ambitious engineering feats ever.

Sixteen nations are involved in the project, led by the United States and Russia. More than 40 manned missions will be required to complete construction.

So far, work has gone smoothly, but project managers are concerned about three pieces of hardware that escaped during earlier spacewalks.

The small pieces of space debris pose no threat to the crew or the mission, but the US says its air force will be monitoring the situation until the components re-enter the Earth's atmosphere and burn up.

A third spacewalk is scheduled for Saturday, with the Endeavour due to return to Earth on Tuesday.