But much remains to be discovered.

NASA lunar scientist Sarah Noble told Reuters separately by phone that it is still unknown how much ice is actually present on the moon and how easy it would be to extract in sufficient quantities to be of practical use.

"We have lots of models that give us different answers. We can't know how much water there is," she said, adding that it would ultimately take surface exploration by robotic landers or rovers, in more than one place, to find out.

Most of the newly confirmed frozen water is concentrated in the shadows of craters at both poles, where the temperature never rises higher than minus 120 degrees.

Although the moon was long believed to be entirely dry or nearly devoid of moisture, scientists have found increasing evidence in recent years that water exists there.

A NASA rocket sent crashing into a permanently shadowed lunar crater near the moon's south pole in 2009 kicked up a plume of material from beneath the surface that included water.

A study published the following year in the Proceedings of the National Academy of Sciences concluded that water was most likely widespread within the moon's rocky interior, in concentrations ranging from 64 parts per billion to five parts per million.

Bridenstine spoke to Reuters about making the next generation of lunar exploration a "sustainable enterprise", using rockets and other space vehicles that could be used again and again.

"So we want tugs that go from Earth orbit to lunar orbit to be reusable. We want a space station around the moon to be there for a very long period of time, and we want landers that go back and forth between the space station around the moon and the surface of the moon," Bridenstine said.

NASA's previous program of human moon exploration ended with the Apollo 17 mission in 1972.

Trump last December announced a goal of sending American astronauts back to the moon, with the ultimate goal of establishing "a foundation for an eventual mission to Mars".

The Trump administration's $US19.9 billion budget proposal for NASA for the fiscal year beginning on October 1 includes $US10.5 billion for human space exploration.

The budget supports development of NASA's new Space Launch System rocket and the Orion spacecraft designed to carry a crew into space. The administration envisioned a SLS/Orion test flight around the moon without a crew in 2020, followed by a fly-around mission with a crew in 2023.

As part of the budget proposal, NASA also is planning to build the Lunar Orbital Platform-Gateway - a space station in moon orbit - in the 2020s.

NASA said the power and propulsion unit, its initial component, was targeted to launch in 2022.

In May, NASA cancelled a lunar rover that was under development, a project envisioned as the first mission to conduct mining somewhere other than Earth.

Reuters