Astronauts aboard the International Space Station were ordered by NASA to shelter in their spacecraft and prepare for potential evacuation on Friday as a Russian crew attempts to fix a worsening leak of air on its portion of the orbital laboratory, NASA said.

The four astronauts of NASA’s Crew-12 mission on the station – two U.S. astronauts, a French astronaut and Russian cosmonaut – got orders from NASA mission control at 9:04 a.m. ET Monday (1304 GMT) to enter their Crew Dragon spacecraft docked to the station and don their spacesuits in case the air leak warrants an emergency evacuation, a NASA official said.

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NASA and Russia’s space agency Roscosmos, the station’s two primary operators, have debated for months over the cause and potential fixes of small air leaks aboard Russia’s Zvezda service module, a key structure of the football field-sized laboratory.

The air leaks have been relatively minor in recent months but escalated on Monday from a pound of air per day to two pounds, according to a senior NASA official who asked not to be named.