The six astronauts at the International Space Station are safe and are taking shelter in the Russian segment after an alarm went off on the US side of the orbiting outpost, NASA said Wednesday.
The US space agency said it could not confirm that an ammonia leak was the cause, as the Russian space agency reported earlier.
"ISS flight controllers are not sure if the alarm was triggered by a pressure spike, a faulty sensor, or a problem in a computer relay box," NASA said on Twitter.
"Crew is safe. No ammonia leak confirmed. Crew responded to coolant loop pressure increases."
Russia's space agency blamed the evacuation on a leak of a toxic substance.
The "toxic substance was emitted from a cooling system into the station's atmosphere" in the US segment of the station, the agency said in a statement.
"At present the American segment has been evacuated and the crew is safely located in the Russian segment," the statement said. The American segment has been sealed off.
A representative of the Russian mission control center told Russian news agencies that the substance is ammonia. The accident could also delay the departure of the US SpaceX cargo ship Dragon, which brought supplies earlier this week.
Fixing the leak may also require an emergency spacewalk, he added.
The International Space Station is a rare area of US-Russian cooperation that has not been hit by the crisis in Ukraine, which has prompted Washington to impose sanctions on Moscow.
In total 16 countries work on the ISS, whose cost is mainly shouldered by the United States.
Since NASA phased out the space shuttle system in 2011, it depends entirely on Russia to send its astronauts to the ISS.