Nasa's Artemis I rocket sits on launch pad 39-B at Kennedy Space Center in Cape Canaveral, Florida, on Friday. AFP

The current launch window for Nasa's Artemis 1 mission to the Moon ends Tuesday and is "definitely off the table," said Jim Free, associate administrator for Exploration Systems Development, at a press conference on Saturday.

The next possible launch window is September 19 to October 4, and failing that, October 17 to 31, Nasa said.

The ability to take off during those windows "will really depend on the options that the team comes back with likely on Monday or early Tuesday morning," said Free.

Millions around the globe tuned in to live coverage and crowds gathered on beaches in Florida on Saturday hoping to witness the historic blastoff of the Space Launch System (SLS).

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But a leak near the base of the rocket was found as ultra-cold liquid hydrogen was being pumped in, forcing a halt.

The Artemis 1 space mission hopes to test the SLS as well as the unmanned Orion capsule that sits atop, in preparation for future Moon-bound journeys with humans aboard.

The first launch attempt on Monday had also been halted after engineers detected a fuel leak and a sensor showed that one of the rocket's four main engines was too hot.

"This is a whole new vehicle, a whole new technology, a whole new purpose of going back to the moon and preparation to go to Mars," said Nasa administrator Bill Nelson. "Yes, it's hard."

The Artemis I unmanned lunar rocket sits on the launch pad at the Kennedy Space Center. AFP

Artemis mission manager Mike Sarafin described the hydrogen leak as "large," and said one of their "leading suspects" was a seal on a fueling tube.

Engineering teams believe they will have to replace the seal, either directly on the launch pad or after taking the rocket back to its assembly building a few miles away.

It was "too early" to entirely rule out a launch before the end of September, said Sarafin, who promised a status update next week.

NASA has previously said that the early October period would be complicated to coordinate because a crew of astronauts will be using the Kennedy Space Center for a rocket launch to the International Space Station.

In addition to the leak, another problem facing the SLS is its emergency self-destruct system.

Designed to explode in case the rocket deviates off course, the system will likely need to be reexamined before the next launch, which can only be done in the assembly building.

Bringing the rocket in and out of the building will take "several weeks," Sarafin said.

Agence France-Presse