NASA has carried out the first-ever medical evacuation of astronauts from the International Space Station (ISS), rushing a four-person crew back to Earth after an in-orbit health emergency.

The Crew-11 capsule splashed down in the Pacific Ocean off the coast of San Diego, California at approximately 3:41am ET on Thursday, ending a roughly 10-hour return flight.

The return was prompted by a medical issue that NASA flight surgeons determined could not be fully treated in microgravity.

On board were NASA astronauts Zena Cardman and Mike Fincke, Japanese astronaut Kimiya Yui and Russian cosmonaut Oleg Platonov. 

The spacecraft fired its engines to deorbit at 2:50am before enduring a fiery re-entry through Earth’s atmosphere, deploying four parachutes to slow its descent into the ocean moments before splashdown.

A recovery vessel with medical personnel retrieved the capsule at sea, conducted immediate health assessments, and transported the astronauts by helicopter and jet to NASA’s Johnson Space Center in Houston for further evaluation.

The American space agency has declined to identify the affected crew member or disclose the nature of the condition, citing medical privacy.

NASA administrator Jared Isaacman decided to bring the crew home early out of an abundance of caution, noting that the astronaut's medical episode was considered 'serious' and would require additional medical care on Earth. 

NASA's chief medical officer, Dr James Polk, said last week they were in stable condition and were not in any immediate danger. 

He noted that the astronaut's medical condition had nothing to do with an upcoming spacewalk on January 8, which was cancelled, or any other operations on board the station. 

NASA is set to host a live conference at 5:45am to discuss the astronauts' return. 

Boats swarmed the capsule as it bobbed on top of the water, which then pulled the craft towards a recovery vessel in the distance. 

This is a developing story... More updates to come.