WASHINGTON (REUTERS) - A US government watchdog on Friday (July 30) sided with Nasa over its decision to pick a single lunar lander provider, rejecting a protest filed by Blue Origin and defence contractor Dynetics.

The companies had challenged the US$2.9 billion (S$3.9 billion) award to Elon Musk's SpaceX for the lander, arguing Nasa was required to make multiple awards.

The Government Accountability Office (GAO) said it "denied the protest arguments that Nasa acted improperly in making a single award to SpaceX."

Blue Origin, the rocket company founded by billionaire Jeff Bezos, said on Friday it remained convinced that there were "fundamental issues" with Nasa's decision, and that GAO was not able to address them "due to their limited jurisdiction."

Blue Origin said it will continue to advocate for two immediate providers as it believes that to be the right solution.

"GAO's decision will allow Nasa and SpaceX to establish a timeline for the first crewed landing on the Moon in more than 50 years," Nasa said in a statement on Friday, adding that sending American astronauts to the moon is a priority for the Biden administration.

Dynetics, a unit of Leidos Holdings, did not immediately comment.

SpaceX did not comment, but Musk sent a tweet saying just "GAO" with a flexed muscle emoji.

Nasa had sought proposals for a spacecraft that would carry astronauts to the lunar surface under its Artemis programme to return humans to the moon for the first time since 1972.

In April, Nasa awarded SpaceX a contract to build such a spacecraft as early as 2024.

Blue Origin had contended Nasa gave SpaceX an unfair advantage by letting it revise its pricing.

On Monday, Bezos offered to cover up to US$2 billion in Nasa costs if the US space agency awarded Blue Origin a lunar landing contract.

In a letter to Nasa Administrator Bill Nelson, Bezos said Blue Origin would waive payments up to US$2 billion, and pay for an orbital mission to vet its technology. In exchange, Blue Origin would accept a firm, fixed-priced contract, and cover any system development cost overruns.

"Without competition, Nasa's short-term and long-term lunar ambitions will be delayed, will ultimately cost more, and won't serve the national interest," Bezos said.

Bezos' offer came six days after he flew alongside three crewmates to the edge of space aboard Blue Origin's rocket-and-capsule New Shepard.