Astronauts hurriedly evacuated the U.S. section of the International Space Station and moved to its Russian module after a coolant problem emerged Wednesday, but Russian and U.S. officials insisted all six crew were not in any danger.

"The space station crew is safe," NASA spokesman Bob Jacobs said.

An alarm indicating a possible ammonia leak in the cooling system early Wednesday prompted the crew to leave and seal off the American module, but further testing had NASA officials thinking it was just a sensor problem, NASA spokesman Mike Curie said.

Still, the crew planned to finish its work day in the Russian segment and sleep there overnight out of caution, Curie said. There's enough room and food for them to stay there a week but that's not likely to be necessary, he added.

The two space agencies differed on exactly what had occurred as the station orbited about 250 miles above the Earth. While the Russian space agency Roscosmos said there was a coolant leak, NASA said in a statement on its online television station there was still "no concrete data that suggests that there was, in fact, an ammonia leak."

Russia's Tass news agency said just about one-third of ammonia was left in the coolant system at the U.S. module and the rest had leaked out. It quoted Roscosmos chief Oleg Ostapenko as saying the situation was still being examined but an "evacuation (of the entire station) is not on the agenda."

NASA said the astronauts evacuated to the Russian module as a precaution.

"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," Jacobs said. "So we protected for the worst-case scenario and isolated the crew in the Russian segment of the space station while the teams are evaluating the situation."

The space outpost is now manned by NASA astronauts Barry Wilmore and Terry Virts, Russians Elena Serova, Alexander Samoukutyaev and Anton Shkaplerov and European Space Agency astronaut Samantha Cristoforetti.