NASA reported a problem Wednesday at the International Space Station but said the crew was not in any danger.

One of two large coolant loops at the ISS -- on which electrical systems ride -- shut down earlier in the day, Josh Byerly, a spokesman for the US space agency, told AFP.

"At no time was the crew ever in danger," Byerly said. "The crew is fine." He said the issue is likely linked to a valve problem.

A lot of the systems in European and Japanese laboratories aboard the ISS have been switched onto the other coolant loop or shut down, he said.

"Some non-critical systems have been powered down inside the Harmony node, the Kibo laboratory and the Columbus laboratory while the teams work to figure out what caused the valve to not function correctly and how to fix it," NASA said in a statement.

"The crew is safe and preparing to begin a normal sleep shift while experts on the ground collect more data and consider what troubleshooting activities may be necessary."

Last month, three astronauts returned to Earth from the ISS after a 166-day mission, bringing the Olympic torch back with them after a historic space walk.