Two astronauts from the US shuttle Endeavour late Saturday successfully completed the first of five scheduled spacewalks aimed at completing a Japanese laboratory at the International Space Station, NASA said.

Tim Kopra, who made his first space walk, and Dave Wolf, an old hand with four walks under his belt, returned to the ISS's decompression chamber and closed the airlock at 2151 GMT Saturday, 37 minutes ahead of schedule, NASA TV reported.

The duo's spacewalk lasted five hours and 32 minutes, US space agency officials said.

"The third and final piece of Japan?s Kibo laboratory was assembled on orbit Saturday, a symphony of robotic and spacewalking performances by the 13 member orchestra aboard the International Space Station complex," NASA said in a statement.

The 1.9-tonne unit known as the Japanese Exposed Facility (JEF), will serve "as a type of porch for experiments that require direct exposure to space," NASA said.

From inside the ISS, astronauts Koichi Wakata and Doug Hurley used the station?s Canadarm2 to grasp the JEF unit and lift it out of the Endeavour payload bay.

"They handed the facility to the shuttle?s Canadarm and moved the station?s arm into position for installation. The shuttle arm handed off the new Kibo component to the station arm, and then the station arm was used to move the new "porch" into position for installation to the Kibo pressurized module," NASA said.

Earlier, on their first full day in space, the Endeavour crew of six Americans and one Canadian tested rendezvous equipment, installed a camera for the orbiter docking system and extended the docking ring that sits on top of the system.

The Endeavour mission aims to help fulfill "Japan's hope for an out-of-this-world space laboratory," as the shuttle delivers state-of-the-art equipment to conduct experiments in the vacuum of space, according to NASA.

Wolf and Kopra had spent the night in the Quest airlock to reduce the preparation time needed for the walk.

The shuttle successfully docked at the space station Friday amid questions about the integrity of the shuttle's heat shield tiles.

However NASA on Saturday said that a close analysis of pictures of Endeavour's heat shield confirmed the absence of any damage.

During the delicate docking maneuver Friday the two space vehicles traveled at 28,000 kilometers (17,398 miles) per hour as they approached each other, giving Commander Mark Polansky a margin of error of 4.5 centimeters (1.8 inches) to complete the procedure, NASA said.

The entry of Endeavour's crew aboard the ISS brought the number of astronauts inside the orbiting space station to a record 13.

Kopra will be staying aboard the ISS, taking over from Japanese engineer Koichi Wakata, who has been in space for 124 days.

The ISS should be completed in 2010, also the target date for the retirement of the US fleet of three space shuttles.