A SpaceX capsule carrying a four-member crew is returning to Earth on ‍the first medical evacuation from the International Space Station.

The emergency flight, which began on Wednesday (US time), was necessitated by an unspecified serious medical condition afflicting one of the ​astronauts aboard.

The Crew Dragon capsule carrying two US NASA astronauts, a Japanese crewmate and a Russian ⁠cosmonaut undocked from the space station and began its descent from orbit, headed for a splashdown in the Pacific Ocean off the California coast early on Thursday.

Live video from a NASA webcast of the departure showed the capsule separating from the ISS and ‌drifting away ​from the orbiting laboratory as the two vehicles soared some 415 kilometres over ‍the Earth, south of Australia.

The astronauts were seen strapped into the Crew Dragon cabin, seated side by side and wearing their helmeted white and black space suits as the undocking proceeded.

NASA officials have not identified which ​of the four crew members is ill, or given more details the nature of the medical emergency, citing privacy ‍concerns.

The crew consists of US astronauts Zena Cardman and Mike Fincke, Japanese astronaut Kimiya Yui and Russian cosmonaut Oleg Platonov. They arrived at the space station following a ​launch to ​orbit from Florida in August.

Fincke, the ​station’s designated commander, and Cardman, the flight ​engineer, had been scheduled to conduct a six-hour-plus spacewalk last week to install hardware outside the station. The spacewalk was cancelled on January 7 over what NASA then characterised as a “medical concern” with an astronaut.

NASA chief health and medical officer James Polk later said the medical emergency did not involve “an injury that occurred in the pursuit of operations”. If all goes as planned, Thursday’s splashdown will conclude what ended up ‍being a 167-day mission.

-AAP