WASHINGTON - US space agency NASA said that one of the International Space Station's cooling systems failed Wednesday but it posed no danger to the crew on board.

"The pump module on one of the space station's two external cooling loops automatically shut down when it reached pre-set temperature limits," NASA said in a statement. "The flight control teams ... suspect a flow control valve actually inside the pump itself might not be functioning correctly."

According to the US space agency, the external cooling loops circulate ammonia outside the station to keep both internal and external equipment cool.

"Although there is no danger to the crew on board, the ground teams did work to move certain electrical systems over to the second loop and some non-critical systems have been shut down while the teams work to figure out what caused the problem and how to fix it," NASA said.

The crew was now preparing to begin a normal sleep shift while experts on the ground collected more data and considered what troubleshooting activities might be necessary, NASA added.

Three astronauts returned to Earth from the ISS after completing a 166-day mission and bringing the Olympic torch back with them after a historic space walk.