Veteran astronauts Butch Wilmore and Suni Williams were due to spend eight days on the International Space Station (ISS), but have been there for more than eight months after their Boeing Starliner spacecraft suffered propulsion problems.

The US space agency decided the Starliner would return home without its crew after carrying out weeks of intensive testing and announced in August that it was tasking Elon Musk's SpaceX with bringing the crew home.

Musk, one of President Donald Trump's closest advisors, committed to bringing them back to Earth "as soon as possible" at the end of January, with Republicans blaming Trump's predecessor Joe Biden for the delay.

A SpaceX mission called Crew-9 saw two astronauts arrive at the ISS in September aboard a Dragon spacecraft, with two empty seats for Wilmore and Williams.

But their return was postponed when NASA announced that Crew-10 who was due to relieve them would not be blasting off until the end of March at the earliest.

The mission is now scheduled to launch on March 12 "pending mission readiness", NASA announced Tuesday, explaining that the change was agreed with SpaceX after adjustments were made to the original plan.

Instead of using a brand new Dragon spacecraft that requires extra processing time, the Crew-10 mission will now use a previously flown one called Endurance.

The astronauts will return to Earth following a handover period of several days, the agency added.

In January, Wilmore and Williams said their spirits were still high, adding that they had plenty of food and were enjoying their time on the space station.

While their protracted stay is notable, it has not yet surpassed Frank Rubio's record-breaking 371 days aboard the ISS, which he completed in 2023 after the Russian spacecraft designated for his return developed a coolant leak.