NASA and Boeing have stalled the Starliner cargo spacecraft to the International Space Station yet again after engineers detected an unexpected problem with its propulsion system.

The Orbital Flight Test-2 (OFT-2) -- the second uncrewed flight for Boeing's CST-100 Starliner spacecraft as part of the US space agency's Commercial Crew Programme -- was earlier targeted to launch on August 3.

However, the mission teams detected indications that not all valves were in the proper configuration needed for launch. They found "unexpected valve position indications in the Starliner propulsion system", NASA said in a statement.

The United Launch Alliance (ULA) has begun removing propellant from the Atlas V rocket.

"NASA and Boeing are standing down from the launch attempt of the agency's Orbital Flight Test-2 to the International Space Station as mission teams continue to examine the cause of the unexpected valve position indications on the CST-100 Starliner propulsion system," the statement added.

Mission teams have decided to roll the Atlas V and Starliner back to the Vertical Integration Facility (VIF) for further inspection and testing where access to the spacecraft is available, while Boeing will power down the Starliner spacecraft. Engineering teams have ruled out a number of potential causes, including software, but additional time is needed to complete the assessment.

Boeing had signed a contract with NASA's Commercial Crew Programme to fly operational missions to and from the space station with Starliner in 2014. However, a series of setbacks have thus far prevented the Starliner from reaching the ISS.

Its debut uncrewed orbital flight mission in 2019 did not go exactly as planned, requiring it to make another try before putting astronauts on board for the crewed flight test.

The second launch has been held up since late last year due to ongoing software checks. It was earlier scheduled for Friday, but was pushed for August 3 after the thrusters on the ISS' new Russian module Nauka Multipurpose Laboratory Module unexpectedly fired hours after docking, causing the station to move out of orientation.

The space station was shoved 45 degrees off track. However, it was repositioned back to normal.

"NASA and Boeing will take whatever time is necessary to ensure Starliner is ready for its important uncrewed flight test to the space station and will look for the next available opportunity after resolution of the issue,a the US space agency said.