US space agency Nasa has blocked Chinese citizens with valid US visas from its facilities - a move that effectively bars them from working at one of the most respected space research centres.

Chinese nationals, who could only work at Nasa as contractors or students contributing to research, found out on 5 September that they had lost all access to Nasa's systems and facilities, Bloomberg news reported, citing sources.

Nasa then confirmed this, saying Chinese nationals would be restricted from using the agency's "facilities, materials and networks to ensure the security of our work".

China's accelerated space programme has alarmed the US and ramped up the race between the two biggest economies.

Chinese astronauts are already excluded from the International Space Station (ISS) because Washington has barred Nasa from sharing its data with China.

The latest restriction from Nasa only adds to the decline in scientific collaboration between the two sides because of national security concerns.

As they compete to gain a technological edge, Beijing and Washington have also grown increasingly wary of each other.

The suspicion has made it harder for some Chinese students, especially those studying science and tech, to get visas, or even enter the US, once they have secured a visa.

There have also been several cases in the US recently of alleged espionage by Chinese nationals, and scientists in particular have come under the scanner.

It's unclear how much notice Nasa gave Chinese nationals who were working for the agency. Bloomberg News reported that they had suddenly found they were denied access to the agency's data systems, and were barred from participating in meetings relating to their work - both in-person and virtual.

Nasa's press secretary Bethany Stevens told news outlets that Nasa had indeed taken "internal action pertaining to Chinese nationals - including restricting physical and cybersecurity access to our facilities".

China has made no secret of its space ambitions, with both Beijing and Washington competing to send their crew to the moon.

"We're in a second space race right now," Nasa's acting administrator Sean Duffy told reporters in a press conference on Wednesday where he spoke about US discoveries on Mars.

"The Chinese want to get back to the moon before us. That's not going to happen."

The director of the general technology bureau of China's Manned Space Agency last year said US concerns were "unnecessary", calling China's space exploration a "collective mission for humanity".

At a US Senate hearing last week, lawmakers stressed the need for the US to put its boots on the moon before China.

"China has made no secret of its goals," said Republican senator Ted Cruz. "[If] our adversaries achieve dominant space capabilities, it would pose a profound risk to America... [the] stakes could not be higher."

The race is not just about getting to the Moon. It's also about who can control its resources once they are there.

The Moon contains minerals, including rare earths, metals like iron and titanium - and helium too, which is used in everything from superconductors to medical equipment.