The first mission in NASA's program to take humans back to the Moon has been delayed until spring at the earliest, as the highly anticipated rollout of space agency's new moon rocket, on its launch pad in Florida for final tests before a first flight, needed to complete more safety checks, the agency said.

NASA, which late last year had targeted liftoff this month for its uncrewed Artemis 1 mission around the moon and back, declined to set a revised launch date, but the delay would preclude a flight before April.

At a briefing for reporters, NASA executives said there were no specific, major difficulties slowing their schedule, but rather a higher-than-usual volume of technical hurdles to clear in preparing a large, complex rocket system for its very first launch.

"It's really what I would call a kind of punch list of a whole bunch of things that we absolutely need to finish up and then we'll be ready to roll the vehicle out," said Tom Whitmeyer, a deputy associate NASA administrator.

NASA officials said workforce and supply disruptions related to the recent omicron-driven surge in COVID-19 infections also were factors in slowing down the work.

At stake is the combined fate of NASA's heavy-lift Space Launch System (SLS) rocket and the Orion crew capsule it will send aloft for the Artemis program, aimed at returning humans to the moon and eventually establishing a long-term lunar colony as a precursor to sending astronauts to Mars.

The United States Apollo program sent six astronauts to the lunar surface between 1969 and 1972, the only crewed spaceflights yet to achieve that feat.

In November, NASA announced that it would aim to achieve the first crewed lunar landing of Artemis, named for the twin sister of Apollo in Greek mythology, as early as 2025.

However, the space agency has several spaceflight stepping stones to meet before it gets there, starting with a successful maiden flight of the SLS and Orion, now in the final stages of pre-launch preparations.

Rollout of the towering spacecraft, a key milestone marking the public's first glimpse of the newly assembled, 36-story-tall rocket-and-capsule vehicle as it is moved, had recently been planned for mid-February.

Under the updated timeframe outlined on Wednesday, the SLS-Orion will be trundled out on a giant crawler-transporter in March – probably around the middle of the month – from its assembly building to Launch Pad 39-B at NASA's Kennedy Space Center in Cape Canaveral, Florida.

Once there, it will take about two weeks for technicians to ready the launch vehicle for a "wet dress rehearsal" that includes fully loading the rocket's fuel tanks with propellant and running through a simulated countdown.

Afterward, NASA will roll the SLS-Orion stack back into the assembly building for a last round of checks before officially setting a new target liftoff date.

In a statement on Wednesday, NASA said it was reviewing launch windows in April and May, but the timeline could slip further depending on the outcome of the dress rehearsal, space agency officials said.