Boeing's troubled Starliner spaceship could be set for further delays after the company said Thursday it was working to solve problems with the propulsion system.

The spaceship's latest launch date on August 4 was cancelled over propulsion issues, and it remains unclear when the next test flight will be scheduled.

"Over the past couple of days, our team has taken the necessary time to safely access and test the affected valves, and not let the launch window dictate our pace," said John Vollmer, Starliner vice president and program manager.

The glitch is the latest to delay development of the capsule, which Boeing built under contract with NASA to ferry astronauts to low Earth orbit following the end of the Space Shuttle program.

Boeing needs to succeed with an uncrewed test flight before flying humans.

SpaceX, the other company given a multibillion-dollar contract by NASA for taxi rides to the International Space Station (ISS), has now undertaken three crewed missions.

Boeing said in a statement it would work with NASA to confirm a new launch date when the spacecraft is ready.

During an initial uncrewed test flight in December 2019, the Starliner capsule experienced glitches with its thrusters.

It did not have enough fuel to reach the International Space Station and had to return to Earth prematurely.