China is bolstering its space capabilities and is using its civilian program to mask its military objectives, the head of the US space agency NASA said Wednesday, warning that Washington must remain vigilant.

"China has made extraordinary strides especially in the last 10 years, but they are very, very secretive," NASA administrator Bill Nelson told lawmakers on Capitol Hill.

"We believe that a lot of their so-called civilian space program is a military program. And I think, in effect, we are in a race," Nelson added.

He said he hoped Beijing would "come to its senses and understand that civilian space is for peaceful uses," but added: "We have not seen that demonstrated by China."

Nelson's comment came as he testified before the House appropriations committee on NASA's budget for fiscal 2025.

He said the United States should land on the Moon again before China does, as both nations pursue lunar missions, but he expressed concern that were Beijing to arrive first, it could say: "'Okay, this is our territory, you stay out.'"

The United States is planning to put astronauts back on the Moon in 2026 with its Artemis 3 mission. China says it hopes to send humans to the moon by 2030.

Nelson said he was confident the United States would not lose its "global edge" in space exploration.

"But you got to be realistic," he added. "China has really thrown a lot of money at it and they've got a lot of room in their budget to grow.

"I think that we just better not let down our guard," Nelson said.