Washington, Sep 4 (DPA) Two astronauts from the space shuttle Discovery completed a more than six-hour-long spacewalk outside the International Space Station (ISS) Friday, installing a new ammonia tank used in the cooling system, NASA said.

US astronaut John 'Danny' Olivas and Swede Christer Fuglesang ended their mission successfully at 0451 GMT. They got started nearly an hour later than planned because of problems with a communications device in Olivas' spacesuit.

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

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

Kevin Ford and Nicole Stott operated the robotic arm.