The test has already faced years of delays and comes at a challenging time for Boeing, as a safety crisis engulfs the century-old aerospace titan's commercial aviation arm.

Astronauts Butch Wilmore and Suni Williams were strapped into their seats preparing for liftoff on Monday night when the call for a "scrub" came.

Ground teams had heard buzzing from a valve that regulates liquid oxygen pressure on the Atlas V rocket meant to propel Starliner into orbit.

United Launch Alliance, the Boeing-Lockheed Martin joint venture responsible for the rocket, initially said launch would be delayed to at least May 10.

But further analysis revealed the valve had sustained too much wear and required replacement. The rocket will be rolled back to its hangar for the repairs.

"NASA's Boeing Crew Flight Test now is targeted to launch no earlier than 6:16 p.m. EDT Friday, May 17," the US space agency said in a blog post.

Wilmore and Williams will remain under quarantine in crew quarters at the NASA Kennedy Space Center, the post added.

NASA is banking on Starliner's success as it hopes for a second commercial vehicle to carry crews to the orbital outpost.

Elon Musk's SpaceX achieved the feat with its Dragon capsule in 2020, ending a nearly decade-long dependence on Russian rockets following the end of the Space Shuttle program.