NASA astronauts Suni Williams and Butch Wilmore will have their return to Earth further delayed until at least late March, the agency said, taking what should have been an eight-day stay on the International Space Station to more than nine months.

The duo had traveled to the International Space Station (ISS) in June for the test mission, but their return was extended by eight months to February, after the Boeing Starliner capsule they arrived on was deemed unfit to return them to Earth.

NASA confirmed that Williams and Wilmore, along with NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov, would return to Earth after the four-member Crew-10 mission, now expected to launch in late March, reaches the space station.

The agency did not specify a precise date for the astronauts' return. Hague and Gorbunov boarded the ISS in September, over three months after Williams and Wilmore.

"Known as a handover period, it allows Crew-9 to share any lessons learned with the newly arrived crew and support a better transition for ongoing science and maintenance at the complex," the agency added in the statement on Tuesday.

The Crew-10 mission was originally scheduled to launch in February, but NASA delayed it to allow the teams more time for processing a new Dragon spacecraft for the mission.