Washington  - Two astronauts from the space shuttle Discovery began a third and final spacewalk Saturday outside the International Space Station (ISS).

US astronaut John "Danny" Olivas and Swede Christer Fuglesang began the scheduled six-and-a-half-hour spacewalk at 4:39 pm (2039 GMT), US space agency NASA said.

They were to deploy an attachment system that will be used to hang spare parts on the station's truss. They were also to replace a device designed to help the station determine its position relative to the Earth.

On Friday, Olivas and Fuglesang completed a more than 6-hour spacewalk outside ISS, installing a new ammonia tank used in the cooling system.

The pair removed an 816-kilogramme 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.

The planned 13-day mission is designed to transport new equipment and experiments to the space station to boost its capacity as an orbiting laboratory.

The crew of six US and a Swedish astronaut brought two "racks" for scientific experiments and a freezer to store experiments set to be sent back to Earth. One of the racks will be used to conduct research on metals, semiconductors, crystal, glass and other materials in microgravity. The other will be used to study liquids and gels in space. (dpa)