(Reuters) - NASA said on Thursday it was moving toward certifying Boeing's ( BA ) CST-100 Starliner for crewed flights later this year or by early 2026 after its inaugural mission to the International Space Station was marred by a system fault, forcing an extended stay.

The agency is working with Boeing ( BA ) to resolve the Starliner's faulty propulsion system, which caused its debut eight-day crewed mission to stretch into a nine-month stay in space for NASA astronauts Butch Wilmore and Suni Williams.

Williams and Wilmore returned earlier this month in SpaceX's Dragon capsule.

Joint teams are working to finalize the scope and timelines for various propulsion system test campaigns and analyses, targeted throughout the spring and summer, as NASA prepares for the Starliner's next flight.

Steve Stich, manager of NASA's Commercial Crew Program, said the flight was likely to be in the timeframe of late this calendar year or early next year.

Boeing's ( BA ) effort to fix the Starliner's faulty propulsion system has added to the aerospace giant's thorny development of a spacecraft that has cost it more than $2 billion.