ISS: where do you go, when you want to go?

Astronauts aboard the International Space Station (ISS) are working to repair one of the two toilet systems on the orbiting space station after it failed overnight, NASA said.

With a record 13 people aboard the station, the main toilet broke down, sending astronauts scrambling to the use backup commodes on the Russian side of the station and aboard the visiting US space shuttle Endeavour.

"Earlier today, the US Waste and Hygiene Compartment (WHC) was shut down after a malfunction," NASA said.

"About six litres of pre-treat flush water is believed to have flowed into the pump separator and into other areas it should not be."

Kylie Clem, a spokesperson for the Johnson Space Centre in Houston, said that two crew members were working to fix the toilet system.

"They have the spare parts in orbit to fix it," she said.

"They are working on it now to hopefully resolve it in the next day or so. It is not an issue, at least in the near term."

There are two remaining toilet systems available for the moment - the second ISS facility and a system aboard the space shuttle Endeavour.

Wastewater dumps could contaminate the station's newly installed platform for science experiments.