Washington, Sep 4 (DPA) Two astronauts from the space shuttle Discovery were working outside the International Space Station Thursday to install a new ammonia tank that is crucial for keeping the orbiting laboratory cool.

US astronaut John 'Danny' Olivas and Swede Christer Fuglesang left the ISS at 2212 GMT on a planned six-and-a-half hour spacewalk. They got started nearly an hour later than planned because of problems with a communications device in Olivas' spacesuit.

The pair will remove an 816-kg ammonia tank from a cargo carrier attached to the station and move it with a robotic arm to its home outside the station. The tank will be the largest item ever moved by astronauts during a spacewalk.

Much of the work involves connecting bolts and cables to hook the tank into the station's cooling system.

Kevin Ford and Nicole Stott will operate the robotic arm.