Launched in June, the Starliner spacecraft carrying Williams and Wilmore to the ISS in a high-stakes mission needed before NASA can certify the spacecraft for routine astronaut flights, Reuters reported.

NASA's associate administrator for space operations, Kenneth Bowersox, during a press conference said, "We don't just have to bring a crew back on Starliner for example. We can bring them back on another vehicle."

"In the case that we have with the Starliner crew flight test, the option to either bring the crew home on Starliner or to bring the crew home on another vehicle, we could take either path," he added.

Bowersox said that more "consensus" is needed among the team at present, adding that they are getting serious about exploring their other options.

Noting that Williams and Wilmore integrated with the Expedition 71 crew aboard the ISS, Bowersox said that though it is helpful to have extra set of hands onboard, they are consuming more supplies meant for the ISS crew.

The mission, which was supposed to last for a week, marked its two-month point on August 6, with NASA assessing all possible options for the safe return of the two astronauts, including the SpaceX Crew Dragon capsule.

The long array of problems including thruster troubles and helium leaks have led to questions over the Starliner's ability to bring Williams and Wilmore back home safely.

The US space agency on Tuesday had said that it is shifting SpaceX's four-person flight, scheduled for this month, to the next month, with a target for September 24 at the earliest.

Steve Stich, program manager of for NASA's Commercial Crew Program, said NASA is considering sending SpaceX's Dragon Crew-9 to the ISS in September, with only four astronauts.

The SpaceX spacecraft would also carry two extra spacesuits for Williams and Wilmore. But they would continue to stay on the ISS till February, 2025, when Crew-9's trip back to Earth is scheduled.

However, Stich noted that the proposed plan has not been formally approved yet. "Our prime option is to return Butch and Suni on Starliner. However, we have done the requisite planning to make sure we have other options open," he added.

Officials said that the delay has been made to provide more time to to analyse the thruster and leak problems that hit Starliner after its lift-off. Notably, this is Boeing's first crew mission flight.

“NASA and Boeing continue to evaluate the spacecraft's readiness, and no decisions have been made regarding Starliner's return,” NASA had said in its statement.