Crew-12 will lift off on a SpaceX Falcon 9 rocket early Wednesday, the US space agency said, with launch time "targeted for no earlier than 6:01 am" local time (1101 GMT).

The confirmation provides a sliver of certainty for the Crew-12 mission, which had faced last-minute rocket problems and staffing changes.

Earlier this week, SpaceX had grounded its Falcon 9 rockets while investigating what the Federal Aviation Administration said was a "stage 2 engine's failure to ignite."

"The Falcon 9 vehicle is authorized to return to flight," an FAA spokesman told AFP Friday.

The temporary pause from SpaceX raised concerns the Crew-12 flight could have been delayed.

The mission will be replacing Crew-11, which returned to Earth in January, a month earlier than planned, during the first medical evacuation in the space station's history.

The scientific laboratory, which orbits 250 miles (400 kilometers) above Earth, has since been staffed by a skeleton crew of three.

NASA has declined to disclose any details about the health issue that cut the mission short.

Additionally, the mission's crew changed in November, when Russian cosmonaut Oleg Artemyev was suddenly replaced by Andrey Fedyaev.

Reports from independent media in Russia suggested Artemyev had been photographing and sending classified information with his phone. Russian space agency Roscosmos merely said he had been transferred to a different job.

In addition to Fedyaev, Crew-12 comprises Americans Jessica Meir and Jack Hathaway and French astronaut Sophie Adenot.

Once the astronauts finally get on board, they will be one of the last crews to live on board the football field-sized space station.

Continuously inhabited for the last quarter century, the ageing ISS is scheduled to be pushed into Earth's orbit before crashing into an isolated spot in the Pacific Ocean in 2030.