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(CNN Business) —  

Hours before Elon Musk is scheduled to host an event about his futuristic rocket concept, the chief of NASA chided SpaceX for delays in its multi-billion contract to fly astronauts to space.

“I am looking forward to the SpaceX announcement tomorrow,” NASA Administrator Jim Bridenstine said Friday on Twitter. “In the meantime, Commercial Crew is years behind schedule. NASA expects to see the same level of enthusiasm focused on the investments of the American taxpayer.”

“It’s time to deliver,” he said.

Musk’s space company has multiple billion-dollar contracts with NASA.

Bridenstine also retweeted a comment from Ars Technica writer Eric Berger that said the statement did not read “as a shot to SpaceX, but rather a reflection of Jim’s desire to see all NASA contractors meet their deadlines for government contracts.”

SpaceX did not respond to CNN Business’ request for comment. NASA declined comment.

NASA set up the Commercial Crew program shortly after the Shuttle Program ended in 2011, with plans to tap the private sector to develop the next spacecraft capable of taking astronauts to and from the International Space Station.

SpaceX and Boeing were awarded $2.6 billion and $4.2 billion contracts, and the expectation was that the companies would have the spacecraft up and running by 2017.

But both companies suffered extensive delays. SpaceX’s Crew Dragon capsule flew an uncrewed demonstration mission in March, during which it briefly docked with the ISS. But a few weeks later, the spacecraft was destroyed during a ground test when a leaky valve triggered an explosion.

Crew Dragon is expected to be further delayed as it works on the issue. NASA and SpaceX have not said when they expect Crew Dragon to be back on the launch pad.

Musk’s focus has been on Starship, a towering rocket that he hopes will one day land the first humans on Mars. SpaceX quickly ramped up development of the spacecraft in 2019. The company built an early prototype at a test site in South Texas, and it completed a few low-altitude tests this summer.

Two new Starship prototypes are now under construction, and Musk plans to give a presentation, updating the public on his Mars travel plans, on Saturday at 7 pm CT.

NASA gave SpaceX its first major contract in 2008 when the startup was strapped for cash and still developing its hardware, and Musk has credited NASA as an essential partner.

Musk said in 2018 that SpaceX was “all hands on deck for Crew Dragon,” and it would be making trips to the ISS by December 2018.

After that, he said, “most of our engineering resources will be dedicated to BFR, and I think that will make things go quite quickly,” he said. BFR was the earlier name for Starship.

SpaceX intends to attempt to fly a Starship to orbit by the end of this year, according to Musk.