Crew members were evacuated from a US segment of the International Space Station after an ammonia leak was suspected.
But Nasa officials now says a sensor problem probably created the false impression of leaking coolant.
The Russian space agency emphasised that the crew members had not been in any danger.
It said that mission control experts in Russia and the US had quickly co-operated to ensure the crew's safety.
The six crew members on the outpost put on breathing equipment and moved into the Russian segment after the alert at around 0900 GMT, closing the hatch to the US side behind them.
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