Four crew members departed the International Space Station (ISS) on Wednesday after a medical issue prompted their mission to be cut a month short – a first for the orbiting laboratory.

A video feed from NASA showed American astronauts Mike Fincke and Zena Cardman, Russian cosmonaut Oleg Platonov and Japanese astronaut Kimiya Yui undocking from the ISS at 2220 GMT, after five months in space.

The U.S. space agency has declined to disclose which crewmember has the health problem or give details about the issue, but it has stressed the return is not an emergency situation.

The affected crew member "was and continues to be in stable condition," NASA official Rob Navias said on Wednesday.

The SpaceX Dragon capsule carrying the four crew members is scheduled to splash down off the California coast overnight at around 0840 GMT Thursday.

"First and foremost, we are all OK. Everyone on board is stable, safe and well cared for," Fincke, the pilot of SpaceX Crew-11, said in a recent social media post.

"This was a deliberate decision to allow the right medical evaluations to happen on the ground, where the full range of diagnostic capability exists. It's the right call, even if it's a bit bittersweet."

The Crew-11 quartet arrived at the ISS in early August and had been scheduled to stay onboard the space station until they were rotated out in mid-February with the arrival of the next crew.

James Polk, NASA's chief health and medical officer, said "lingering risk" and a "lingering question as to what that diagnosis is" led to the decision to bring back the crew earlier than originally scheduled.

American astronaut Chris Williams and Russian cosmonauts Sergey Kud-Sverchkov and Sergei Mikaev, who arrived at the station in November aboard a Russian Soyuz spacecraft, are remaining on the ISS.

The Russian Roscosmos space agency operates alongside NASA on the outpost, and the two agencies take turns transporting a citizen of the other country to and from the orbiter.

Located some 400 kilometers above Earth, the ISS functions as a testbed for research that supports deeper space exploration, including eventual missions to return humans to the moon and onward to Mars.

The four astronauts being evacuated had been trained to handle unexpected medical situations, said Amit Kshatriya, a senior NASA official, praising how they have dealt with the situation.

The ISS is set to be decommissioned after 2030, with its orbit gradually lowered until it breaks up in the atmosphere over a remote part of the Pacific Ocean called Point Nemo, a spacecraft graveyard.