Merritt Island, Florida - NASA on Tuesday postponed the departure of astronauts bound for the International Space Station on a SpaceX flight due to ongoing issues with Boeing's Starliner spacecraft.

The Starliner, which brought two NASA astronauts aboard the ISS in early June, has been docked for the past two months due to anomalies detected in its propulsion system during its flight, with the mission's return date increasingly uncertain.

The SpaceX flight was supposed to carry out the next group of four ISS crew members, dubbed Crew-9, in mid-August, to relieve the current foursome known as Crew-8 currently aboard the space station.

Given the stranding of the two additional astronauts, however, NASA has had to devise a plan to find another way to bring them back to Earth.

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The Crew-9 flight, now scheduled to liftoff by September 24 at the earliest, could bring back the two stranded astronauts and only two Crew-8 members, giving Boeing more time to fix issues with its spacecraft.

"This adjustment allows more time...to finalize return planning for the agency's Boeing Crew Flight Test currently docked to the orbiting laboratory," the US space agency said in a blog post.

The Starliner, which also had helium leaks detected during its voyage, could also return unmanned if safety issues are not resolved.

"NASA and Boeing continue to evaluate the spacecraft's readiness, and no decisions have been made regarding Starliner's return," the post continued.

NASA is planning a press conference on Wednesday to provide updates on both Crew-9 and the Starliner spacecraft.