Astronauts at the International Space Station have taken shelter after an alert indicated there could be a dangerous ammonia leak on board the orbiting laboratory, according to NASA.
The US space agency said on Wednesday its early analysis identified that the alert was likely to be a false alarm.
NASA's Butch Wilmore, the station commander, together with Terry Virts, a flight engineer with NASA, and Samantha Cristoforetti, a flight engineer with the European Space Agency, abandoned the US side of the orbital outpost after an alarm sounded around 4am (09:00 GMT).
The trio joined three Russian crewmates on the Russian side of the station, which is a partnership of 15 nations, overseen by the US and Russia.
The precautionary move came as ground-control teams detected increased pressure in a water line in one of the station's two cooling loops, a possible indication that ammonia may have leaked into the line.
The Russian space agency told news outlets in Moscow that the cause of the alert was a toxic ammonia leak, but Rob Navias, NASA mission commentator, said there was no hard data to indicate a leak had occurred.
"It's becoming a stronger case that this is a false indication, which is great news," Jim Kelly, an astronaut at
mission control in Houston, radioed to the crew shortly before 8:30am (1330 GMT).
Kelly told the crew that mission control is assessing the situation to determine if the three could return to the US section of the station later on Wednesday.
Ammonia is used in the cooling and heating systems on the station.