A problem in the U.S. segment of the International Space Station on Wednesday prompted its six-person crew to quickly lock it up and move to a Russian module, but they aren't in danger, Russian and U.S. official said.
"The space station crew is safe," NASA spokesman Bob Jacobs said.
Russia's space agency Roscosmos said in a statement that a "leak of harmful substances from the cooling system" prompted the crew to isolate the American module.
"The crew is safe and is in the Russian segment now," it said in a statement, adding that mission control experts in Moscow and Houston quickly and efficiently cooperated to ensure the crew's safety.
While Roscosmos said positively that there was a leak, NASA said it was still unclear whether it actually had occurred.
"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 while the teams are evaluating the situation," Jacobs said.
The Tass news agency reported that just about one third of ammonia was left in the coolant system at the U.S. module and the rest has leaked out.
It quoted Roscosmos chief Oleg Ostapenko that the situation was still being examined, but "evacuation is not on the agenda."
The space outpost is manned by NASA astronauts Barry Wilmore and Terry Virts, Russians Elena Serova, Alexander Samoukutyaev and Anton Shkaplerov and European Space Agency astronaut Samantha Cristoforetti.