The astronauts are back in the American side of the International Space Station.

Early Wednesday, the crew fled to the Russian segment after an alarm indicated a possible toxic leak. But NASA later said there was no leak of ammonia coolant and a computer problem likely set off the false alarm.

By Wednesday afternoon, the astronauts had returned to the U.S. side.

When the alarms went off around 4 a.m., the crew followed emergency procedures — slapping on oxygen masks, taking cover in the Russian quarters, then sealing the hatches between the U.S. and Russian sides.

At the same time, flight controllers at NASA's Johnson Space Center in Houston turned off non-essential equipment.

Commander Butch Wilmore, an American, and his crew were busy dealing with supplies from the newly arrived SpaceX capsule when the ammonia-system alarm sounded.

The complex, which orbits 420 kilometres above Earth, has never had to be abandoned during its 14-year-plus occupation by astronauts. On occasion, crews have had to seek shelter in their Soyuz capsule "lifeboats" because of close shaves with orbiting junk, in case a quick getaway was needed. This time, the astronauts went into one of the three Russian modules, but not the two docked Soyuz capsules.

The space station experienced an ammonia leak in 2013.

Canadian astronaut Chris Hadfield, a former commander of the ISS, said an ammonia leak is one of the three major incidents that the station occupants train to confront. The other two, he tweeted, are "fire/smoke [and] contaminated atmosphere/medical."