Officials did not provide details of the medical event but said the unidentified crewmember is stable. They said it did not result from any kind of injury onboard or from ISS operations.

NASA chief medical officer James Polk said "lingering risk" and a "lingering question as to what that diagnosis is" led to the decision to return early. Officials insisted it was not an emergency evacuation.

The four astronauts on NASA-SpaceX Crew 11 -- US members Mike Fincke and Zena Cardman along with Japan's Kimiya Yui and Russia's Oleg Platonov -- would return within the coming days to one of the routine splashdown sites.

Amit Kshatriya, a NASA associate administrator, said it was the "first time we've done a controlled medical evacuation from the vehicle. So that is unusual."

He said the crew deployed their "onboarding training" to "manage unexpected medical situations."

"Yesterday was a textbook example of that training in action. Once the situation on the station stabilized, careful deliberations led us to the decision to return Crew 11... while ensuring minimal operational impact to ongoing work aboard."

- 'Trained professionals' -

The four astronauts set to return have been on their mission since August 1. Such journeys generally last approximately six months, and this crew was already due to return in the coming weeks.

Officials indicated it was possible the next US mission could depart to the ISS earlier than scheduled, but did not provide specifics.

Chris Williams, who launched on a Russian mission to the station, will stay onboard to maintain US presence.

Russians Sergey Kud-Sverchkov and Sergei Mikaev are also there.

NASA had previously said it was postponing a spacewalk planned for Thursday due to the medical issue.

Astronauts Fincke and Cardman were to carry out the approximately 6.5-hour spacewalk to perform power upgrade work.

Continuously inhabited since 2000, the ISS functions as a testbed for research that supports deeper space exploration -- including eventual missions to Mars.

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.