"We have not yet received final confirmation that a cooling agent has been leaked into the station's atmosphere. So far we only have information from the cooling circuit that the pressure of ammonia has fallen," the US space agency source said.
NASA also said the station's atmosphere is being checked from a distance.
Russia's space agency, Roscosmos, reported earlier that ammonia from an air conditioning unit in the American segment of the ISS was discharged into the station's atmosphere. The American side of the ISS is currently isolated, with its crew safe and taking refuge on the Russian side.
Expedition 42, currently on board the space station, comprises six specialists from the United States, Russia and the European Union. It includes commander Barry Wilmore and flight engineer Terry Virts from NASA; flight engineers Elena Serova, Alexander Samokutyayev and Anton Shkaplerov from Roscosmos; and flight engineer Samantha Cristoforetti from the European Space Agency (ESA).
One American and two Russian crew members — Scott Kelly, Mikhail Kornienko and Gennady Padalka — are expected to replace Wilmore, Serova and Samokutyayev in March for Expedition 43.