Updated 2.30pm

An engine problem has delayed the launch on Monday of NASA's most powerful rocket yet on an uncrewed test flight to take humans back to the Moon and eventually to Mars.

Blastoff, which had been planned for 8.33am (12.33pm GMT), was put on hold because of a temperature issue with one of the four engines on the 322-foot (98-meter) Space Launch System (SLS) rocket, the US space agency said.

Tens of thousands of people - including US Vice President Kamala Harris - gathered along the beach near the Kennedy Space Center in Florida to watch the launch, which comes 50 years after Apollo 17 astronauts last set foot on the Moon.

The goal of the flight, dubbed Artemis 1, is to test the SLS and the Orion crew capsule that sits atop the rocket. Mannequins equipped with sensors are standing in for a crew for the mission.

Overnight operations to fill the rocket with more than three million liters of ultra-cold liquid hydrogen and oxygen were briefly delayed by a high risk of lightning, though it was a "go" after an hour. 

Around 3am, another hiccup emerged: a potential leak was detected during the filling of the main stage with hydrogen, causing a pause. 

After tests, the flow resumed.

"The leak is at an acceptable level and we have returned to fast fill operations," NASA tweeted. 

But NASA engineers later detected a problem with the temperature in one of the four engines and put a hold on the countdown. NASA had a two-hour window Monday in which to carry out the launch.

The massive orange-and-white rocket, which has been sitting on the space center's Launch Complex 39B for more than a week, is not able to take off in case of rains and storms. 

Soon after 2.33pm GMT it was announced that the launch has been scrubbed, with September 2 being the earliest alternative flight date.

Extreme temperatures

During the 42-day trip, the Orion capsule will orbit the Moon, coming within 60 miles (100 kilometres) at its closest approach, and then fire its engines to shoot out 40,000 miles - a record for a spacecraft rated to carry humans.

One of the mission's primary objectives is to test the capsule's heat shield, which at 16 feet in diameter is the largest ever built.

On its return to Earth's atmosphere, the heat shield will have to withstand a speed of 25,000 miles per hour and a temperature of 5,000 degrees Fahrenheit (2,760 degrees Celsius) - or half as hot as the Sun.

The dummies aboard the spacecraft will record acceleration, vibration and radiation levels.

The craft will deploy small satellites to study the lunar surface.

A complete failure would be devastating for a program costing $4.1 billion per launch that is already years behind schedule.

Life on the Moon

Monday's launch is "not a near-term sprint, but a long-term marathon to bring the solar system and beyond into our sphere," said Bhavya Lal, NASA associate administrator for technology, policy and strategy.

The next mission, Artemis 2, will take astronauts into orbit around the Moon without landing on its surface. The crew of Artemis 3 is to land on the Moon in 2025 at the earliest.

And since humans have already visited the Moon, Artemis has its sights set on another lofty goal: a crewed mission to Mars.

The Artemis program is to establish a lasting human presence on the Moon with an orbiting space station known as Gateway and a base on the surface.

Gateway would serve as a staging and refuelling station for a voyage to Mars that would take a minimum of several months.