NASA said Thursday that it will bring four astronauts aboard the International Space Station back to Earth more than a month earlier than planned because of a medical issue - the first such evacuation in the space station`s 25-year history.

Citing medical privacy concerns, NASA did not provide additional details about the issue, including the identity of the affected crew member, the nature of the medical problem or its severity. Agency officials said the situation is stable, however.

NASA Administrator Jared Isaacman said at a news briefing that the astronauts will return home in the coming days. The agency has not yet given a precise timeline for undocking or landing.

"After discussions with chief health and medical officer Dr. JD Polk and leadership across the agency, I`ve come to the decision that it`s in the best interest of our astronauts to return Crew-11 ahead of their planned departure," Isaacman said.

Isaacman added that further updates will be provided over the next 48 hours.

The group leaving the International Space Station are NASA astronauts Zena Cardman and Mike Fincke, Japanese astronaut Kimiya Yui and Russian cosmonaut Oleg Platonov. The astronauts, known as Crew-11, arrived there in early August and had been expected to stay aboard the orbiting laboratory until late February.

Polk said that the situation is stable and that the evacuation is not considered an emergency. Rather, he said, the decision was made to err on the side of caution for the affected astronaut`s health and welfare.

"We have a very robust suite of medical hardware onboard the International Space Station, but we don`t have the complete amount of hardware that I would have in the emergency department, for example, to complete a workup of the patient," Polk said. "And in this particular incident, the medical incident was sufficient enough that we were concerned about the astronaut that we would like to complete that workup."

NASA first made the medical issue public Wednesday, when it announced that it was postponing a spacewalk Cardman and Fincke were scheduled to conduct Thursday.

After Crew-11`s early departure, NASA will face several weeks with just one of its astronauts onboard the space station to oversee U.S. science experiments and operations - flight engineer Chris Williams, who launched aboard a Russian Soyuz spacecraft on Nov. 27. Russian cosmonauts Oleg Platonov, Sergey Kud-Sverchkov and Sergey Mikaev are there with him.

The next crew members are scheduled to launch to the ISS in mid-February, but Isaacman said NASA will evaluate whether to bump up that mission, known as Crew-12.

This week`s drama in orbit is the first major test for Isaacman: He was sworn in on Dec. 18.