MOSCOW - A coolant leak at a US 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, the Russian space agency said.

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.

Mission control experts in Moscow and Houston quickly and efficiently cooperated to ensure the crew's safety, the agency said. It added that Houston specialists are now analyzing the situation with the US segment.

The Interfax news agency quoted Russian Mission Control as saying that there was an ammonia leak at the US segment.

A spokeswoman for NASA office in Moscow said she had no immediate comment.

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.