Image caption Emergency spacewalks are being planned

The crew of the International Space Station have been forced to reduce power after half the cooling system suddenly shut down over the weekend.

Nasa officials insisted the three Americans and three Russians aboard were not in danger.

Urgent spacewalk repairs are being discussed for this week.

Without thermal controls, temperatures on the ISS's Sun-facing side can soar to 121C (250F), plunging to minus 157C (-250F) on the dark side, Nasa says.

"There might be a comfortable spot somewhere in the middle of the station, but searching for it wouldn't be much fun," a statement on its website adds.

The station is now operating on a single string, the Associated Press reports, and has no safeguard in case of further cooling system failures.

Alarms sound

Trouble arose on Saturday night when one of the two ammonia-fed cooling loops shut down, triggering alarms throughout the ISS

ISS crew members Tracy Caldwell Dyson

Shannon Walker

Doug Wheelock

Fyodor Yurchikhin

Mikhail Kornienko

Alexander Skvortsov

The two ammonia lines ensure that all the station's electronic equipment does not overheat.

Astronaut Tracy Caldwell Dyson set in motion equipment shutdown procedures and, with crewmate Douglas Wheelock, installed a jumper cable to keep all the rooms cool.

The Global Positioning System circuit, several power converters and a set of devices that route commands to various pieces of equipment were switched off.

Two of the four gyroscopes - part of the space station's pointing and navigating system - were initially shut down but the crew installed a jumper cable to bring up a third gyroscope, leaving the station in a much more stable position, AP says.

Flight controllers tried to restart the disabled ammonia pump early on Sunday but the circuit breaker tripped again.

Any repairs later this week almost certainly will involve replacing the faulty ammonia pump, a difficult job that would require two spacewalks, AP adds.

"It's pretty clear that we're going to want to have a course of action to take as quickly as possible," Nasa spokesman Rob Navias said at the Johnson Space Center in Houston, Texas.

"This is not something we want to linger over."

Two spare pumps are stored on the outside of the station. Two of the Americans on board are already scheduled to conduct a spacewalk on Thursday for routine maintenance.

No space shuttle visits to the ISS are planned before November.