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.
Jim Kelly from Nasa mission control in Houston told the crew: “We’re still trying to figure out exactly what happened. We’re not entirely convinced that this is an ammonia leak.”
A spokesperson for the space agency, Bob Jacobs, added that there was “no concrete data that suggests that there was in fact an ammonia leak”.
“We saw an increase in water loop pressure, then later saw a cabin-pressure increase that could be indicative of an ammonia leak in the worst case scenario, so we protected for the worst case scenario and isolated the crew is the Russian segment of the space station,” he said.
Suspected ammonia leaks on the outside of the space station occurred in 2013 and 2009.
The outpost is manned by Nasa astronauts Barry Wilmore and Terry Virts, cosmonauts Elena Serova, Alexander Samoukutyaev and Anton Shkaplerov and European Space Agency astronaut Samantha Cristoforetti.
The Expedition 42 crew had been awake for about two hours before the alarm sounded, and was at work unloading the SpaceX Dragon cargo carrier which arrived days ago with more than 2.5 tonnes of supplies and science experiments.