Workers detected a problem with helium flow to the massive SLS rocket that will "take the March launch window out of consideration," Issacman said in a post on X.

"I understand people are disappointed by this development. That disappointment is felt most by the team at NASA, who have been working tirelessly to prepare for this great endeavor," Isaacman said.

"During the 1960s, when NASA achieved what most thought was impossible, and what has never been repeated since, there were many setbacks."

The towering SLS rocket and Orion spacecraft will be rolled back into the Vehicle Assembly Building at the Kennedy Space Center in Florida to investigate the technical issues and make any necessary repairs, Isaacman said.

A complete briefing will follow in the coming days, he added.

The highly anticipated Artemis 2 mission, lasting about 10 days, will see three Americans and one Canadian fly around Earth's satellite.

It would be a huge step towards Americans once again setting foot on the lunar surface, a goal announced by President Donald Trump in his first term.

On Friday, NASA had set March 6 as the earliest possible launch date.

The US space agency hopes to put humans back on the Moon as China forges ahead with a rival effort that is targeting 2030 at the latest for its first crewed mission.

China's uncrewed Chang'e 7 mission is expected to be launched in 2026 for an exploration of the Moon's south pole, and testing of its crewed spacecraft Mengzhou is also set to go ahead this year.

NASA surprised many late last year when it said Artemis 2 could happen as soon as February -- an acceleration explained by the Trump administration's wish to beat China to the punch.

But the program has been plagued by delays.

The uncrewed Artemis 1 mission took place in November 2022 after multiple postponements and two failed launch attempts.

Then technical problems in early February -- which included a liquid hydrogen leak -- cut short a so-called wet dress rehearsal for the Artemis 2 launch. That was finally completed earlier this week.

The wet dress rehearsal was conducted under real conditions -- with full rocket tanks and technical checks -- at Cape Canaveral in Florida, with engineers practicing the maneuvers needed to carry out an actual launch.

NASA is hoping that the Moon could be used to help prepare future missions to Mars.