A serious medical condition with crew member aboard the international space station has led Nasa to bring astronaut and three crewmates back to Earth months earlier than planned the first emergency return in orbiting laboratory 25 year history senior space agency said.

Nasa administrator Jared Isaacman told media on Thursday in short notice press conference in Washington that he and medical official made decision to return astronaut whom he did not identify because the capability to diagnose and treat this property does on live on international space station.

The NASA official did not identify which of Crew 11 mission four astronauts was experiencing the medical problem or describe its nature citing crew member privacy.

This is not emergency evacuation.

This is first early evacuation in history of ISS which has been continously inhabited since year 2000.

The four person team is called Crew 11 and consist of NASA astronauts Zena Cardman and Mike Fincke Kimiya Yui from Japan JAXA space agency and Russian cosmonaut Oleg Platonov.

One American astronaut will remain aboard the ISS official say.

It will be accompanied by Two Russian cosmonauts.

Dr James Polk Nasa chief health and medical officer told media that this was the first time in Nasa over 65 year history that mission would comeback early due to medical problem.

Crew 11 launched to ISS in August last year on SpaceX Crew Dragon and were expected to remain in orbit for almost six months coming back almost next month after being replace by other four person crew few days earlier.

The ISS has basic medical equipment supplies and system that let doctors on Earth talk privately to astronauts in space assess their condition and treatment much like safe video or phone consultation with general doctors.

Returning the four person crew early could delay some experiments and maintenance goals until new crew arrive next month according to Dr Simeon Barber a space scientist at Open University.

The space station is big complex feat of engineering it is designed to operated by certain minimum level of crew he added.

He said that the remaining crew would probably be forced to dial back on some of experimental work and focus more on housekeeping and keeping station healthy waiting  for full complement of crew to be restored.

Spacewalks are arduous and risky mission that require months of training involving bulky spacesuits and coordinated instruction while tethered to ISS.

An update was expected within 48 hour on timeline of astronauts return the Nasa officials added.

On Wednesday Nasa cancelled a spacewalk due to take place on Thursday when two astronauts were set to step outside the ISS citing medical worries.