Two US astronauts stranded for months on the International Space Station will remain there at least until late March, NASA said Tuesday as it announced another delay in the mission to bring them home.

Veteran astronauts Butch Wilmore and Suni Williams arrived at the ISS in June aboard Boeing's Starliner spacecraft, and were due to spend eight days on the orbiting laboratory.

But problems arose with the Starliner's propulsion system during the flight there, so NASA opted for a big change in plans.

After weeks of intensive tests on the Starliner, the space agency decided to return it to Earth without its crew, and to bring the two stranded astronauts back home with the members of a SpaceX mission called Crew-9.

Crew-9's two astronauts arrived at the ISS aboard a Dragon spacecraft in late September, with two empty seats for Wilmore and Williams. The plan was for all four to return home in February 2025.

stranded astronauts on the iss
NASA's Boeing Crew Flight Test astronauts Butch Wilmore and Suni Williams. (NASA)

But NASA said Tuesday that Crew-10, which would relieve Crew-9 and the stranded pair, would now launch no earlier than March 2025, and both teams would remain on board for a "handover period."

"The change gives NASA and SpaceX teams time to complete processing on a new Dragon spacecraft for the mission," NASA said in a blog post.

The bottom line is that Wilmore and Williams will spend more than nine months in space, rather than eight days as initially planned.

SpaceX, the private company founded by billionaire Elon Musk, has been flying regular missions every six months to allow the rotation of ISS crews.

© Agence France-Presse