Washington DC - Two US astronauts who arrived at the International Space Station aboard Boeing's Starliner will have to return home with rival SpaceX, NASA said Saturday.

"NASA has decided that Butch and Suni will return with Crew-9 next February, and that Starliner will return uncrewed," NASA administrator Bill Nelson told reporters.

NASA astronauts Barry "Butch" Wilmore and Sunita "Suni" Williams blasted off for the ISS on Starliner on June 5 for what was meant to be an eight-day stay.

But their return has been delayed by thruster malfunctions that came to light during the first crewed mission to the ISS by the Boeing spacecraft.

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In a Wednesday press conference ahead of the SpaceX announcement, NASA official Ken Bowersox said the astronauts were "making the best" of their extra time aboard the ISS "but I'm sure they're eager for a decision, just like the rest of us."

Bowersox added that "our big concern is having a successful deorbit burn."

Wilmore and Williams returning with SpaceX marks the biggest setback to date for Boeing's space program.

Both Boeing and SpaceX were awarded multibillion-dollar contracts in 2014 to provide the US space agency with rides to the ISS following the end of the space shuttle program in 2011, which left the US reliant on Russian Soyuz rockets to ferry its crew.

SpaceX succeeded in its first crewed test in 2020 and has carried dozens of astronauts since.