Washington DC - NASA said on Friday the Boeing Starliner's return to Earth from the International Space Station, which was scheduled for next week, has been delayed again.

The delay of the rescheduled June 26 return will allow more time for the review of the thruster malfunctions and helium leaks that caused the first delay, it said.

Starliner's first crewed mission to the ISS, which had initially been scheduled to last about eight days, has been extended to a date yet to be determined.

The spaceship blasted off from Florida atop a United Launch Alliance Atlas V rocket on June 5 following years of delays and safety scares, as well as two aborted launch attempts that came as astronauts were strapped in and ready to go.

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"NASA and Boeing leadership are adjusting the return to Earth of the Starliner Crew Flight Test spacecraft," NASA said in a blog post late Friday.

"The move off Wednesday, June 26, deconflicts Starliner's undocking and landing from a series of planned International Space Station spacewalks while allowing mission teams time to review propulsion system data," it said.

NASA's Commercial Crew Program manager Steve Stich said: "We are taking our time and following our standard mission management team process."

"We are letting the data drive our decision-making relative to managing the small helium system leaks and thruster performance we observed during rendezvous and docking," he said.

Stitch insisted that Starliner, which is crewed by two astronauts, Butch Wilmore and Suni Williams – both ex-US Navy test pilots – is "performing well in orbit while docked to the space station".

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NASA said the crew was not in a hurry to leave the station because it is well stocked with supplies and the "station's schedule is relatively open through mid-August".

The initial ISS docking was delayed by more than an hour after some of Starliner's thrusters that provide fine maneuvering initially failed to kick in.

It was known there was one helium leak affecting Starliner before the launch. While non-combustible, helium provides pressure to the propulsion system.

However, other leaks emerged during the flight.