Two astronauts from the US shuttle Endeavour on Thursday began the first of a series of spacewalks to install Japan's maiden laboratory at the International Space Station, according to NASA.

Astronauts Rick Linnehan and Flight Engineer Garrett Reisman emerged at 8:18 pm Central Daylight Time (0118 GMT Friday) to begin maneuvering phase one of the laboratory out of Endeavour's payload bay and attaching it to the orbiting station, NASA said.

During the walk expected to last six and a half hours, the pair also were to begin assembling a Canadian component for the robotic arm named Dextre, a dextrous manipulator used for delicate tasks normally reserved for an astronaut on a spacewalk.

Lead spacewalker Linnehan and Reisman spent the previous night in a decompression chamber to purge nitrogen from their bodies, the National Aeronautics and Space Administration said.

As Linnehan and Reisman floated outside the ISS in the brief glow of an orbital sunset, astronauts inside prepared to use the station's robotic arm to move the initial lab module into place.

The Japan Aerospace Exploration Agency's Kibo laboratory is a micro-gravity research facility which aims to open a vital new stage in deeper space exploration.

With the installation, Japan gains a foothold on the ISS alongside the United States, Russia and Europe, whose laboratory Columbus was delivered to the station in February.

The first Kibo stage being delivered is ELM-PS, a 4.2-ton logistics module that will serve as a storage facility once Kibo is installed in its entirety.

Kibo's pressurized module, the larger cylindrical heart of the lab which will allow astronauts to work and conduct experiments in a shirt-sleeves environment, is to be transported to the ISS on space shuttle Discovery due to launch May 25.

The final Kibo installment, an inter-orbit communications system unit called the Exposed Facility, is due for delivery in March 2009.

The mission's fourth spacewalk will see specialists Robert Behnken and Mike Foreman conduct a space shuttle thermal shield repair demonstration using a device similar to a caulk gun.

Protecting the shuttle's thermal tiles is of particularly concern since the space shuttle Columbia disaster in 2003.

Columbia disintegrated re-entering Earth's atmosphere, killing all seven on board, because its thermal shield had been damaged when it was struck by a piece of debris during launch.

The 16-day mission for Endeavour, whose crew includes Japanese astronaut Takao Doi, is the longest mission at the space station and will conduct five space walks totaling some 30 hours of work.