Washington (AFP) - The mystery surrounding the fate of a secret military satellite deepened Thursday when the Pentagon refused to answer even simple questions about whether the mission to launch it had gone awry.

On Sunday, private space firm SpaceX blasted a Falcon 9 rocket from Cape Canaveral, Florida carrying the secret government satellite, known as Zuma.

US media this week reported that the billion-dollar payload did not make it into orbit and was presumed to have been lost.

SpaceX said Tuesday that the rocket worked fine, but its statement left open the possibility that something could have gone wrong after the launch.

When asked at a press briefing if the Pentagon considered the launch a success or a failure, two officials declined to provide any information whatsoever because of the classified nature of the mission.

"I would have to refer you to SpaceX, who conducted the launch," Pentagon spokeswoman Dana White said.

When pushed on the matter, fellow spokesman Lieutenant General Kenneth McKenzie said: "I'm done. We're not going to be able to give you any more information."

Northrup Grumman, the maker of the payload, has said it was for the US government and would be delivered to low-Earth orbit, but offered no other details.

SpaceX has launched national security payloads in the past, including a spy satellite for the National Reconnaissance Office, and an X-37B space plane for the US Air Force.

The CEO of SpaceX is Elon Musk, the South African-born inventor and entrepreneur who is also behind electric car-maker Tesla.